While it may seem that way at lower levels, the math in the DMG makes it clear.
Base AC for creating a monster CR 1/4-3 is 13. With a +0 to your main attack stat, you will hit 50% of the time, +2 60% and +3 65%. So it's clear that the game as designed requires only a +1 to you main attack stat to hit more often than you miss.
A CR12 Monster base AC is 17, and yes, a +4 is needed there to keep that 60% hit rate, so a +3 is the breaking point to hit more often than you miss.
And just to be clear, I'm not saying one way or another is the 'right' or 'preferred' way to play, I'm only examining why the designers are okay with choices existing that would prevent a player from getting a 65% chance to hit at level 1 (along with a host of choices that do allow you to hit that higher number). Like magic items, choosing a background at creation that gives you a 16 is a bonus over the baseline assumption of the game.