Sure there are a few companies that care about marks, but most don't

By later co-ops, that's true, but for first co-op that's not as accurate.

but most don't (and that includes the awesome places everyone wants to work at)

Bloomberg, Google, Wish are 3 off the top of my head, and everyone I know who worked at Jane Street had a pretty high average (tbf n = 2 for them).

That isn't to say they won't interview people with lower marks, but high marks will impress them enough to get a job.

I can count on one hand the number of reputed companies that actually care about marks.

SWE class profile 2018 shows clearly that for first co-ops there's a very strong correlation between marks and compensation. There's also an increasing trend in lower bound of compensation with increase in marks

it's true that they aren't as big a deal as other things, marks do have a (admittedly decreasing) impact