I mean, unless you're double-checking everything with a guide or website, you won't allways know if you caught everything in a given area, at least not during a first playthrough.
So what if you move on thinking you caught everything but you missed some mon with a 2% chance? Or the opposite, what if you wasted half an hour on the grass expecting something rare would eventually appear but there was nothing else? Not to mention other reasons why many wild Pokémon can be easily missable (like you're playing at nighttime and the Pokémon only appears during day, or a specific weather condition, or only available under special circumstances like SOS battles, etc.)
What I do is looking on websites for potential canditates for my team, see which Pokémon can be found in the game I started playing, and take notes on where/when they can be found so I don't miss the ones I care about. But catching everything has never been exactly an intuitive task, I see no point in completely cleaning up every area before allowing myself to proceed since I'm not a collector, and even if I was I still wouldn't see the point because you can't just complete the dex as you go anyway for many reasons.
Rarer Pokémon also usually become way easier to find later in the game, so why bother searching for a rare early game Golbat encounter when eventually you'll find Golbat everywhere in late game caves? Many Pokémon can be eventually found a lot easier and more naturally if you just keep playing normally.
