Too much effort. They mostly go for fire-and-forget strategy. Any additional processing time means less sent emails = less profit.

What? It's minimal effort. 'Found' a gmail address? Drop everything after the + sign.

Spammers spend a lot of processing time on their emails (e.g. testing them against spam filters to tweak them to get through), so this extra step is so small as to be insignificant.

its so well known and common, I would be surprised if spammers already weren't ignoring dots and the plus for gmail addresses