It's just RNG, even back on story maps you can go 5+ maps without getting any SGs if you are unlucky.

I must say that even if these events are RNG hell, at least i haven't had to make any changes to my standard team for eons now and can use it for every damn map in existence. It's kind of funny to see people all go on about how you need different speed teams all the time. I keep all my girls at different speeds regardless.