If the actual universe is infinite, wouldn’t it contain all possibilities?
Not necessarily. A collection may be infinite whilst making systematic exclusions. Consider the set of even numbers. It's infinite in number, but it doesn't contain all the numbers. Granted, the odd numbers never really had a chance, in this artificial case. So perhaps the thought is instead that an infinite expanse will contain every finite pattern that it possibly could. This seems more intuitive. But in fact this too turns out to be false.
Consider an infinite sequence of coin-flips. Suppose it is a fair coin, so for each individual flipping event, there is a 50/50 chance of it landing heads or tails. So one obvious possibility is for a "tails" event to occur. But is it guaranteed to occur, at some point in the infinite sequence? Well, no. There is, after all, some (infinitesimal) chance that the coin will land heads on every single flip whatsoever! In this case, all the other possibilities that might have obtained, won't. So, infinity is no guarantee of actualizing every possibility.