Indeed, Houdini attempted a learning feature, but it was so bad nobody used it and it was removed from later versions.carldaman wrote:Stockfish and Houdini would be worth much less in comparison, if that were to happen.
I guess a problem is Stockfish Learning is GPL'd, by definition you can't plug it in your engine and sell it, Houdini or Komodo can't just copy and paste the code because it'd be illegal to sell the engine, they'd have to open the source which isn't going to happen!
Why don't other top free engine authors implement learning? They're too busy chasing elo, what with them not even fully implementing UCI? Chances are they haven't implented the Exclude Moves feature, you're lucky if they have implemented MultiPV, and some engines like Booot don't even allow you to make it play at fixed depth! So Learning is at the bottom of the list.
By the amount of elo they're able to add in a short frame (Laser seems to be joining this group soon), adding learning would have them stop adding elo for the month it takes them to add it, so they'd have to choose between a 3250 engine with learning, or a 3200 engine without.
The choice is clear because outside of engine tournaments nobody really uses weak engines. Again, with Laser, I didn't even know the engine existed and wouldn't have know if it didn't make the jump in strength, and I'll probably be checking out the next version.
On the other side, YACE has learning, Spike has learning, RomiChess has learning, and I haven't used them for years, so it's kind of difficult to make everyone happy...