Many paid links are eventually ignored by search engines, meaning you paid for "juice" that doesn't actually help.
Here is an informative story about two business owners that illustrates why buying backlinks is a "high-risk, low-reward" gamble. The Tale of Two Sites buying backlinks good or bad
Real human reviewers at Google can manually penalize your site. Many paid links are eventually ignored by search
Leo wanted results immediately. He went to a marketplace and bought a "Platinum SEO Package" for $500. Within a week, he had 2,000 new backlinks from various blogs and forums. Leo wanted results immediately
Slowly, reputable coffee websites began linking to her guide because it was actually useful. These were high-authority backlinks that Google trusts. By month six, Maya reached page one. Unlike Leo, her position was stable and immune to algorithm updates because her links were earned, not bought. Summary: Why it’s usually "Bad"