On: Could a modern chess engine on a Pentium have defeated Kasparov in 1996?

Why is the time per move fixed? A chess engine can, of course, do much better by deciding itself how much time to allot to each move.

Why would it be better for an engine to use less than the maximum time it has available per move?

Because in competitive chess games, the time limit is usually 40 moves in 90 minutes. You don't have a fixed amount of time per move. It's up to you how you want to spend that time budget.

Exactly, and budgeting your time well is a very important skill of professional chess players.