That's just it, if you're concentrated in one area, the enemy just takes control of whatever area you aren't there, and can easily group up and mow your entire team down because they know where you are (as everyone is grouped).
They usually will take baron if you don't take some risk by trying to stop them. They know you're grouping and turtling, so they go for baron. If you don't take risks by solo pushing out all the lanes, they will grab uncontested drags and barons.
After baron, they do as you said; they split up. But you can't leave any lane alone, because, with baron buff, they will just take whatever lane they're in down in an instant. Once they get baron, it's over. Some champs don't even need baron. Most champs can mow down turrets really quickly, because of how weak they are lategame. You can't do anything; if you group up in one lane, you lose two inhibs by the time you finish the job. If you spread yourself thin, you get dived. The only way out is to do an all-or-nothing gambit, or, better yet, not fall so behind early on. The state of the game doesn't allow comebacks as much as it used to, even if you play lategame champs.
No, you don't get picked up one by one. You take risks, you understand the risks, and you play safely around them.
But don't just group up like a bunch of idiots in one lane, trying to form a giant mass to protect yourselves. That's useless, and puts up a flag for the enemy to converge and kill off the entire team to end the game.
Ward (especially key objectives), shove out all the lanes, but play cautiously. Ideally, you send the immobile targets to farm the waves closest to the base, and the most mobile targets or most fed ones deeper.
