I cannot account for that one particular Scarlatti / Horowitz rendering, but since Horowitz did, for me, about the
sine qua non of Scarlatti renditions on a piano, your questions in this line are beginning to irk me more than a little, honestly.
To
'tell the truth' I'm beginning to form the notion of one with the attention span of a six week old puppy
Scarlatti sonata, 'Formless' indeed! Just because a piece is not in a form with which you are terribly, or at all, familiar does not merit its being called 'formless.' Those Scarlatti sonatas are 'sonatas' -- one trouble being is that some heavy handed German musicologist, and the public in general, all seem to think the ONLY real 'sonata form' is as used by Beethoven, and that those are somehow the ultimate, Ideal, and only acceptable shape of the form,
which is a complete load of hooey! (Then, of course, to be inconsistent much of the general public 'bail' on Luigi's two-movement sonatas, and those no longer clear cut 'traditional' or even some of the more extended forms of the middle ones, and have no truck with the 'weirdly shaped' final few of the 32.)
Some of the traditions you are listening to just don't match your listening habit(s).
Tradition is tradition: habits are habits: habit has nothing to do with tradition.
If Luigi is one of your main home boys, try the Diabelli Variations, lasting near one hour long.
Test your attention span: see what you find.
That said… and to be non-tactfully truthful,
"As a listener, I don't do solo instrumental literature that well."
I really cannot make it through a professional concert-length piano recital (ca. 1 hour, 20 min.) without beginning to twitch a little, no matter that the instrument affords the widest range of any other single acoustic instrument, nor how much incredible tone color the executant can manage (that too is a huge range of itself), because I begin to crave at least one other timbre.
(Heavy Irony -- I am first a pianist, second a composer who has made a near fifty minute long suite for solo midi piano: go figure, though the suite is titled in a way which clearly indicates two parts, since I expect a listener of that piano suite to want / need to break it into halves, with an intermission.)
I have not 'had the problem' when it is music by Schumann, Beethoven, Mozart, Messiaen, Debussy, Chopin, and a few others, where the music has me so attentive that I forget to 'want' another instrument, and that includes the long sets of variations, a full presentation of the complete etudes (Chopin, Debussy), etc. Those composers have never 'failed' me that way.
But the same wanting happens when I listen to the Bach solo 'Cello suites, another single instrument with an exceptionally wide range. After solo piano or solo 'cello, the downward spiral of my lack of tolerance over length of duration accelerates rapidly:. solo flute, bassoon, for more than a few minutes, regardless of how brilliant the music is… I have to steel myself and dwell on the music more than that irksome single timbre, with or without all its glorious colors, including extended techniques.
I'm happiest with instruments, then, in combination, two, preferably at least three, or more.
I've provided you with a quasi-reasonable 'rationale,' or 'tic' -- not a good excuse -- for your lack of being able to listen to solo piano for long. BUT, you have also declared a flaking out during the slow movements of some pieces, orchestral, with 'all the colors.'
It is possible you are yet another fan who happens to be a much more casual listener, and you prefer to 'cherry pick' the movements you like most. That is not criminal, though some might find it 'offensive,' lol.
Also possible: the more casual listener has not yet developed a more concentrated listening habit for music past a lesser duration. (Yes, listening to music, involves conditioned habit, western classical and much of Jazz, a lot of conditioned habit if you get really 'involved.')
That is not to say you get less in the way of pleasure or its emotional import, but that western classical (and Jazz) commands of the listener an aggressive intellectual attentiveness in order to make sense of the whole, which is otherwise a whole lotta notes without much 'meaning.'
If you are slipping into the music like you might slip into a hot tub of water, or even a hot tub with jet circulation, then that is your "difficulty with the music" -
you are being far too passive a listener. The only remedy for that is to sit up, listen more attentively, and not take the attitude of 'just letting it wash over you.'
There is nothing at all wrong with using music and 'letting it wash over you,' but it seems between the lines of your questions you are thinking there might be more to it than you've found, and that is the best remedy I can think of to offer, aggressive ears connected to a very actively listening mind --
even if you are not so immediately entertained or pleasured by what you are hearing.
Non-passive entertainment
means you are pretty much half of what is doing the work. If that is even slightly surprising to you, there is your obstacle.
P.s. if you are of the 'big splash' kind of taste, i.e. "O Fortuna' but not the rest of the work, that is rather like not seeing past the makeup on a beautiful and intelligent woman, you haven't even gotten to the skin, as Hilltroll's analogy has it, let alone 'inside' to the heart and soul. Usually, that is a matter of 'youth' of one sort (you could be 47 and 'new' to classical, ergo in that regard, 'young.' And youth takes care of or sheds -- or accepts -- its problems as it grows.)