The team is just one variable in what makes a startup successful. It happens to be the most important variable IMO. But collecting data wouldn't help here, as success may be attributed to a founders network or availability of capital or just the idea being so great e.t.c. too much to account for scientifically. And not repeatable it seems.
They bias towards multiple founders because running any business on your own is hard. You can collect this anecdote from almost every solo founder. It requires such a breadth of skills that is rarely in just one person. You don't have time to specialise in multiple non related areas. You need help.
What can you do about it? Successful solo founders tend to hire very quickly so if you're doing well and you're in a position to start hiring, do it, then apply to accelerators or whatever with your hired team. Or don't and just continue working on your business like a normal person.