If you have 10,000 followers but only 10 likes per photo, Instagram's algorithm detects that your audience is fake. This signals that your content is not engaging, causing Instagram to stop showing your posts to real people (the "Shadowban"). Wccom — Spy
Many of these sites harvest data. If you are asked to log in with your Instagram credentials, the site could potentially scrape your personal information, DMs, or password. Transpirella Work Direct
The followers you gain are often bots or inactive accounts. Instagram regularly purges fake accounts. You might gain 100 followers today, but next week you could lose all of them.