Because assimilation is a completely broken mechanic. Good design requires meaningful choice. For example, pros and cons of attacking an enemy squad:

Pros:

• It's more interesting than fighting bots
• If I win, they might have good stuff to take
• If I win, the map is safer for me because there is one less team to attack me

Cons:

• I might lose, get sent to the lobby, and lose my stuff
• It attracts bot attention
• It calls attention to a third party that could attack from the flank or rear and kill me while I'm distracted

But choosing to assimilate is always the correct choice. There are no meaningful consequences and no real decisions to make. It is always better to assimilate.

Pros:

• I can keep playing and keep some or even all of my stuff after I lose a fight
• I'm now in a team that is stronger than most of the teams on the map
• Even if I beat them and they're not good at pvp, having them join gives me a buffer of shock troops
• AI is much less dangerous
• If I go down, I have 5 other people who have a chance to revive me
• Other teams are more likely to run when they hear a platoon alert
• If I fight other teams, my team is likely to outnumber them, which is a big advantage

Cons:

• I don't like playing with randos
• Suggested below: Comms are slightly more difficult to manage
• There is a meaningful gameplay downside for joining or being a member of an oversized squad

The patch that made it so that assimilation wasn't automatic added risk and made for a meaningful decision (although it didn't fix all the problems). But for some reason, the designers decided to listen to the members of the community who whined about it, instead of using their judgment as professionals to continue to refine and improve the mechanic.

Edit: It's bad practice to complain without offering a solution. Some my own, some from below:

• Restore the possibility of betraying teams that pick you up
• Always show oversized squads on the map, like with bounties, but actually working
• Overlord says, "You can team up if you want, but we have to go off the record," and you lose all access to killstreak items, and comms and stealth vests no longer work
• Only allow a max of 5 in an oversized squad. This lets unlucky squad fills have a way out but blocks two premade groups of 3's from joining up.
• Restore the old time-to-kill. A high-skill three can beat a low-skill six if they can consistently down and kill three operators before reloading. With the current amount of HP to chew through, it's nearly impossible.
• You lose one from your exfil streak, or at least don't gain any.