"How can the Beatles be so good (I'm not a big Beatles fan, but think they are/were good) and Paul McCartney (as a solo act) be so bad?"
I hear Paul still puts on a good live show!
Even in their prime, no solo Beatle could be equated with the Beatles as a group. That was a unique synergy that could be emulated but never equaled consistently.
However good or bad McCartney is/was as a solo act really has no relevance to what the Beatles were.
There are a few McCartney albums that are worth mention that can claim some level of artistry approaching that of the Beatles albums. "Ram", "Band On the Run", "London Town" and "Tug Of War" are the ones I would cite personally though opinions will vary widely regarding the merit of much of McCartney's solo stuff.
Can't comment on most of his recent albums though...haven't heard much of them.