Because I was playing 5e. If combat is not an option, then I've tossed the majority of the ruleset. If I'm going to do that, I should be playing a different game. I mean, I can do it, but that's pretty far past the point of violating the game's core assumptions.
To expand, if I let my players build characters, a large part of which is how well they do in combat, and then tell them that doesn't matter, I'm not adhering to the social contract that was established when we started with 5e. I'm being a dick.