You can ingest enough grapefruit to affect sensitive medications, it's just much much harder. In fact, you can also drink enough orange juice or other citrus juice to reach the threshold. It even takes several glasses of juice in a day to have an interaction with most drugs it does interact with, for example 24oz has absolutely no effect on blood warfarin levels, but 50oz does.

This is strictly speaking about metabolism, some drugs absorb faster/slower in a highly acidic stomach so for drugs like viagra, you're getting more absorption + less metabolism which is a compound problem. You can also have drugs that absorb less and need to be metabolized to work, which is closer to taking no drug than taking too much drug, in terms of treatment.