The meticulously maintained grand pianos at major performance halls are certainly tuned accurately. As are the pianos of individuals who are careful to keep their instruments in top condition.
Unfortunately, a great number of pianos are not kept well. A lot of school and home pianos take a beating and are only serviced when they get unplayable. For these pianos, tuning is a battle. It's not like a guitar where you have just a few strings and you can simply dial them in to their correct frequencies. Pianos have way too many strings and various concessions have to get made to make the instrument sound in tune with itself. On an abused piano, it may be a difficult enough task to do this; trying to also get it all the way up to concert pitch may be too difficult to be worth it. If the piano is only going to be used as a solo instrument (common in the home), why does it matter if A=435 instead of A=440?
Not to mention that A=440 isn't even a universal standard.