Why is the piano part more difficult than the string parts?

[judiciary]

I've been looking for something to play in a piano trio with two friends. It seems like there are plenty of piano trios where the piano part is as difficult as some solo repertoire (e.g. Schubert, Mendelssohn, Ravel), while the string parts are significantly dumbed down. I feel like I would put in an equal amount of work to learn either of Schubert's as for a Beethoven sonata, while my friends can verify that the violin/cello parts are easier than most solo pieces. Talking to other chamber musicians, it seems typical that piano trios/quartets/quintets are hardest on the pianist.

Why is this? Any historic reason?? Was there some tradition where every composer was a virtuoso pianist while the string players were random people they picked off the street?

I will probably pick something easy, so not any of the trios I mentioned. I ask out of pure curiosity

[jamienc]

My first thought would be because the piano is by nature an “orchestral” instrument, capable of all kinds of texture and capable of much more range than any one string instrument from a practical standpoint. Add to that the fact that we can play many more notes simultaneously and you probably have a good reason for the piano parts to be a bit more involved. I remember years ago when I played the Rachmaninoff cello sonata with a friend, and after our first rehearsal she said she felt bad because she felt she wasn’t participating nearly enough! 😁

[liszt-and-the-galops]

Depending on what time period the pieces are from, at least part of that is because Liszt wrote the piano part  .
Jokes aside, the piano is definitely a more orchestral instrument. I once heard it said that the reason the piano is so popular is because "it allows a single person to play an entire orchestra." I'm willing to bet that another reason is that there are a lot of things that are nigh-impossible on many instruments that a skilled piano player can accomplish with ease (e.g arpeggiated chords, volume, etc.) Because of this, any of those parts have to be played by the piano.
Hope this helps!

[lilycolli89]

Did you find it? In piano trios, the piano often takes on a more important role in musical content and texture. It plays an accompaniment role and leads the melody, harmony, and rhythm. Meanwhile, the violin and cello parts often have a complementary or dialogue role with the piano, sometimes playing a more fundamental role than the main melody line.