I am curious as to how bug hunters / pen testers use DirBuster and GoBuster without getting their IPs banned all the time (which is why I am asking)?
It isn't possible to simply just "avoid the WAF" it is doing its job. Well, with the exception of specific payload bypasses. But that isn't what your question is asking.
In terms of "bypassing" the WAF in relation to something like directory brute-forcing the best course of action you can take is as suggested by Schroeder, set your scanning speed lower than the threshold the WAF is set. For example, if the WAF is configured to start dropping packets when a single IP sends 100 requests or more in a 5 minute window. Then you would need to set your scanning to send no more than that amount.
To find the sweet spot you simply start your scanning quite aggressively (depending on the environment) and lower it gradually until the point where your requests go through. I should note that these days, WAFs can have some pretty granular rules so this really only applies to the classic scenario of dropping traffic when it is too frequent.