Likely because a lot of SEO tactics are not necessarily things that hurt the quality of the site for the user. Using the right heading tags, image alt texts, meta data/schema markup, a good title and meta description etc are all SEO tactics, and they're also all things that help the user experience.

Similarly, a lot of inbound/offsite SEO tactics are theoretically things that help the user as well. Providing content people want to link to, getting authorities to link to said relevant content, etc are all things a user would appreciate.

Using SEO tactics as an anti signal would boost poorly designed sites, inaccessible sites, etc. What really needs to be done is something that filters out sites creating thin content just for the purposes of getting traffic, and that's harder to filter out.

"The site you're after" will also try SEO tactics, because they also want to be seen. And if not using SEO gave you a better rank, then that would also become an SEO tactic.

Turtles all the way down.