I love this question and I am looking forward to see what hedonic utilitarians come up with here. This has similar vibes to computronium thought experiments but better. Thanks for pointing this question out to me :)

To keep it really simple from my perspective only

1. Because I don't aim for total increasing utility. Other things matter besides pleasure and pain.
2. Because I think the probability that bugs feel pain in a meaningful way is so low that it constitutes a pascalls mugging

I’ve just noticed (should have asked LLMs earlier :) this has already been suggested by others as a form of repugnant conclusion problem in totalist views. 

For reference, linking Jeff Sebo’s paper below but he is not the only one who talked about it.

https://philpapers.org/rec/SEBTRC