It's too easy for people to give 1 star or 5 stars and move on. When they need to write a review i.e. explain their rating, it becomes more meaningful because people can read the reason for their rating.
On the Steam platform, the marked leader by far, you only have 1 and 5 star ratings in a way. You can vote up or down. And ratings with little text are drowned out by their helpfulness system. You will not read the reviews of a game with 1000+ reviews. You will see a few of them, and those will have "helpful" text.
On Itch with mostly anonymous ratings most games barely have ratings at all. It is not the problem that people rate meaningless, it is the problem of them not rating at all. But as soon as you have a game with many ratings, it also evens out. Speaking about rating things you did not download. For a system with so little restrictions, in my observation rating manipulation is very rare.
You can scroll past https://itch.io/feed?filter=ratings
Most, like 9 out of 10 have no text and the rest a one liner. Reviews are very rare.
So would we see even less ratings or more, if reviews would be mandatory and public? There are reasons to expect an increase and other reasons for a decrease. And would the average text get more meaningful?