Probably because we'd rather play real humans?
I used to mix playing against computer and playing against humans, back with some old version of Pro Deo. I'd learn the engine's tactics and use them against the human opponents, and I'd try to use the tricks I learned when defeating the engine (at low depth) against humans.
Back then I had a rating of about 1100.
Now I have a rating of about 1750.
What did I learn from the 650 elo jump?
Well, first, that there's only so much that engines are going to teach you. Once you reach diminishing returns, it's better to play twice against humans, than once against human and once against computers, this is because...
Human chess and engine chess are entirely different games.
Against humans you can hang up the queen, and the opponent may miss it. Against an engine the game may basically be over after you blunder a pawn.
Conversely, you're going to defeat the human because they'll play natural mistakes, and you're going to defeat the engine when it plays moves you'll never find in real games.
One thing that I suferred from, from fighting with engines was a "defeatist" attitude. When you blunder against an engine, you will see that the game only gets harder and harder, and eventually become hopeless. I don't ever recall defeating an engine when down on material...
I applied this to human chess, so when they were up two pawns, or the exchange, or a piece, I'd just go and resign.
But, the truth about human chess opponents, that engines can't teach you, is:
That the human elo fluctuates a lot during the game.
What you need to do is keeping your elo as constant as possible, so you punish the opponent whenever they fluctuate and play weaker than you.
I have defeated several +1900 elo players in recent weeks, and I can say that they play like this:
+2800 elo Opening (they can play the opening just as well as Magnus Carlsen)
+3500 elo Once out of book, you may see Alpha Zeroesque moves that could take Stockfish hours to find
+1900 elo These guys are though to beat because they play at their rating in average, so if you're a +1700 you're playing 200 elo below them, in average.
+1700 elo But once in a while they will play just like you, specially in tactically quiet positions.
+1500 elo Specially when they see they're not making progress against you, they may start sacrificing pawns, and other material, to create tactics potentials, and if you're not solid enough, you will die in some attack, but these are unsound moves, there's some continuation that defeats their moves that is there, and if you find it, you'll get an advantage
+1200 elo The reason these guys aren't rated higher is because they get destroyed by 2100 elo players just like they destroy you, and it's when they play these terrible moves that you're not exploiting, but a stronger player would. These are the kind of moves that may be deep, or may only be found by an engine in postmortem, but these are patterns that repeat, and you gain elo as you learn to recognize them.
+0 elo 1900 elo players hang pieces, or the queen, or allows a mating attack, all the time, and you may not notice because of panic, or because of you have your plan set, and the opponent's blunder doesn't stop it, so you continue with your plan, instead of taking advantage of the blunder.
Something else that humans do, that engines don't, is playing in a segment of the board, like, a part of the board that only exists, and the rest of it doesn't matter. The opponent may be down a full piece on the entire board, but up a rook in the segment that s/he's playing, so if you play on the segment that they're ignoring, you just give them extra tempo, so you should be aware of the segment of the board that matters. Engines always play the full board.
A main difference between Romi learning and human learning is pattern recognition. If you defeat a human by having a Bishop on g2 and making a discovered attack on their a8 rook by moving away the knight, that's a pattern they will recognize, in future games they may even play c6 instinctively to protect themselves against it, after you put a bishop on g2.
Romi, and other learning engines, can't learn such patterns, and if they have learned a lot in a variation, and you play against them with exactly the same principles (that is, all their learning would apply to the current position) but with a pawn on a3, they've never seen this before, so the learning does nothing.
You have to ask why humans play chess, and why would they choose to play it agaisnt an engine or a human.
Currently there's places like lichess that allow tournaments in where it's very easily to play opponents 200 or 400 elo stronger than you, that will teach you how strong humans play, allowing you to see their holes, to learn how to defeat them in the future, as you can check offline what mistakes they made, learn the patterns that would work in the future, and maybe next time, trap their queen (this isn't theory,
I have done it.)
In contrast, playing Romi, or any other engine, and beating it, is only going to help you against engines, but has no practical value on games against humans, so this is my guess about why a learning feature doesn't make it more enticing to play against one.
