In react, everything is a function and all your code runs on every render unless you specifically tell it not to. This really encourages a certain style of writing code and provides a lot of guarantees about safety and scope.
Solid is literally the polar opposite, your code runs once, and only the parts that you specifically make reactive are reactive. This allows for much finer-grained updates and performance.
The mental models are so different because they are optimized for different things. React allows for developer sanity (cue all the people who worked on one lousy react app telling me that it doesn't) and Solid optimizes for speed and simplicity.
Solid is very well designed though. One of the features I loved is that Ryan specifically built it to work as possible to vanilla html/js so you can copy old stack overflow answers.