Historically, most party-based cRPGs based on Dungeons & Dragons generally favor a martial-heavy party. About the only exception I can think of is Icewind Dale 2, and that's because you are encouraged to run multiple Clerics in lieu of any Fighter class, since Clerics can do everything and are the premier "whack things" class. (In turn, Icewind Dale 1 is the opposite and the game really wants you to run a bruiser company with token spellcaster support).

Multiple reasons have been touched on in this thread. I would add that a lot of items possess strong on-hit spell like effects that might be unheard of in a standard tabletop loot table, and any notion of balanced wealth goes out of the window when you can tackle any place whenever you want and all the sidequests have preplaced goodies.

Most of the encounters are really just about opening a can and dealing a lot of damage to it, and if you want to do that without asking a lot of questions, then cRPGs just don't ask any. Any situations you might have read about in tier lists about how a dragon may fly and render your fighter useless just don't apply.