The core issue is that it actually is impossible to maintain full democracy under capitalism. Even under perfect direct democracy with no lobbies and full representation those with the means to promote their voice louder will do so.
And if you have big money (which some will, because the more money you already have, the easier it becomes to hoard even more), you can fund projects that will have to promote you in return, skewing the voting process.
In reality though, political lobbying, corruption, etc. are omnipresent, and extremely hard to combat, because it's in the logic of capitalism to accumulate wealth at all costs, legal or otherwise.
Now, I'm not saying socialist societies are totally devoid of corruption and self-interest, but they at least have mechanisms in place to curb it.
Capitalism is not aimed at increasing people's wellbeing, it's aimed at pursuing profit, and people's wellbeing is fundamentally secondary. If putting people in worse conditions increases profits, this will eventually be done. Socialism, on the other hand, declares people's equality and wellbeing as the core priorities. Resources should be spent in a way that benefits most people.
For those who just want to click the link:
https://en.wikipedia.org/wiki/List_of_most-streamed_artists_on_Spotify