The reason is simple: we KNOW that the issues we currently suffered are enabled by capitalism. Because capitalism's core is "money equals power" and "more money equals more power". This incentivises behaviour like Shell's/BP's/Exxon's to do climate change research, learn that they're fucking the planet over, and then proceed to bury that research under a sleuth of fake, corporate-sponsored "research" stating otherwise.

People like me say "no true socialism/capitalism" because it's true. It's also true that we don't know what issues that system would cause. Maybe Universal Basic Income does collapse a society because laziness ultimately wins over all other values? We don't know!

What we do know is that every time a country/society tried implementing socialist/communist solutions to capitalism-induced problems, the results were exceptional. Look at Finland's homelessness statistics. Look at Baltimore and it's crime rates. Every single time a 4-day work week was tested anywhere on the planet, it was touted a massive success that boosted productivity and happiness of employees. Etc., etc., etc.