Because the company that runs Yahoo (Verizon Media) doesn't care to spend the money it would take to clean up this site, because YA doesn't make enough money for them to warrant such an expenditure. It would require their staff monitoring the site (or at least opening up a heavily monitored email avenue for people to report those types of abuses), staff to handle the abuses/abusers, and it would have to be ongoing because suspending or deactivating a troll account doesn't stop them from creating another one.
If the site was a good money maker, it might be worth it to them, but as Yahoo found many years ago, even at YA's height, when it was really popular and super active, it cost them more than they deemed feasible to have live staff to handle all the reports and complaints. They pared down the staff and started making a LOT of the functions automated so staff was only involved when the system wasn't able to handle the issue.
Don't expect that to change.The options these days are to tolerate the abuse (report it as much as possible, but also accept that it's not going away) or leave the site.