Because if you "waste" a week writing something yourself, nice & lean, you'll be told you've fallen for NIH and probably YAGNI as well, and get your hand slapped. But if you introduce a 3,500-strong dependency tree that'll waste a person-month spread across the team, over a year, plus make the product perform worse and waste god-knows how many person years for your users, to do the same thing... nobody complains as long as whatever you imported has lots of github stars and (ideally) looks good on a résumé.

Repeat for several decisions and soon you have tens of thousands of deps.