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That may have been on purpose.  Dicking around with time travel would create a complete mess, full stop.

I know it's a comedy and you are not supposed to think about,  but if you think about it...

If old Biff went back in time and changed the future,  how did Old Biff get back to the original future where Doc Brown and Marty were looking for the DeLorean to go back to the original present.

Then they leave Marty's girlfriend in the alternate hell hole present while they go back to the 1950s to change the past AGAIN,  how did Marty's girlfriend get from the alternate present to the brand new present?

Oh never mind.  Just don't think about it.

I think I saw a website once that said 21 different time-lines and/or alternate realities were created during the events of the BTTF trilogy.

Not a problem for me, I rarely think at all.

The worst one was Escape from the Planet of the Apes. With just a few lines of exposition, they expect the audience to believe that Cornelius and Zira somehow anticipated that Taylor would make a heat of the moment decision to blow up the Ape planet at the end of the previous installment, and believed it with such conviction that they were inspired to retrieve his spaceship from the bottom of Lake Powell. Then they were somehow able to repair it back to working order, even though they were from a primitive society of apes that didn't even have cars. On top of that, they were able to pilot the ship out into space, and backwards in time to 1973? GTFO. I'd like to see that movie. 

Besides. If it was so easy to retrieve the ship and fix it that a couple of horse-riding apes could do it, then why in the Hell didn't the three astronauts just do that in the first movie, instead of living out the rest of their days on the Ape Planet? 

The worst one was Escape from the Planet of the Apes. With just a few lines of exposition, they expect the audience to believe that Cornelius and Zira somehow anticipated that Taylor would make a heat of the moment decision to blow up the Ape planet at the end of the previous installment, and believed it with such conviction that they were inspired to retrieve his spaceship from the bottom of Lake Powell. Then they were somehow able to repair it back to working order, even though they were from a primitive society of apes that didn't even have cars. On top of that, they were able to pilot the ship out into space, and backwards in time to 1973? GTFO. I'd like to see that movie.

Besides. If it was so easy to retrieve the ship and fix it that a couple of horse-riding apes could do it, then why in the Hell didn't the three astronauts just do that in the first movie, instead of living out the rest of their days on the Ape Planet?

Speaking of unnecessary sequels

Any of the time travel movies get into conundra, if that's a word, I think, for good reason.

Imagine you could travel to a planet 100 light years away at "warp speed"and had a telescope powerful enough to see Earth.  You'd be looking 100 years into the past.  So, you return to Earth at "warp speed", and when are you?

How would the Earth be in the same place every time? Was the flux capacitor supposed to be the center of the Universe, with everything else moving relative to the DeLorean?

Obviously the Earth's gravity field holds the car in the same geographic location in relation the the center of earth as the DeLorean travels through time.