It is not up to the teams, let alone the players. The problem is MLB's protocols - testing, contact tracing, their definition of "close contact" - were inadequate, and better protocols, as we are seeing now, are basically unworkable.
What we've also seen is that the players can't be relied on to never break distance while they're at the ballpark. And even if the Marlins or Braves started this by going to a bar, which is definitely one of the worst things you can do, the next time it could just be from a family member or extremely unlucky "essential" errand.
The system was supposedly set up so teams could still play games even if they lost 5 or even 10 players for a while, and teams not playing the same number of games was also inevitable. But now that it has happened both things seem untenable, because one assumes it's gonna happen again.
Like I said, I wouldn't underestimate MLB's incomptence or trust Manfred, but here's what was said about Sunday:
Why the Marlins’ game on Sunday was able to take place after three players were found to have tested positive the morning of the game: “Well I think really what happened was there was testing on Friday and one positive on Saturday. [There was] testing again on Saturday and the three additional positives on Sunday. What then happened under the protocols was we did contact tracing on all four positives. There was, I think, a small number of players who met the [Centers for Disease Control and Prevention] guidelines. They were quarantined. We ordered additional testing. We did symptom checks. We did temperature checks and decided to proceed with the game on Sunday.”
Manfred said that simply being teammates and in the same clubhouse alone don’t necessarily meet the requirements for close contact that would require someone to have to quarantine.
“There’s a distance requirement as well as a duration requirement,” Manfred said.
“All I can tell you is that there was plenty of communication before that game,” Klentak said. “There were others at the league level who knew about the positive test before we did. So the determination was made at that point that it was safe.