1. Tracking is wrong
You will scale toward a number that is not real, faster.
2. Losing on rank, not budget
You are already in the auctions and losing them. More money changes nothing about relevance.
3. The campaign is below break-even
Scaling a loss is still a loss, in larger numbers.
4. Operations cannot take it
Orders you cannot ship become refunds and bad reviews, and both come out of the same margin that was paying for the ads.
Budget is the right answer when a profitable campaign is capped. That is the one case, and it is also the one most often ignored.