As others mentioned, GET requests are supposed to be idempotent, i.e., you can send the same request 100 times and get the same response (with no server side-effects) 100 times.

GET requests are also easier to be abused in Cross Site Request Forgery (CSRF) attacks. Modern countermeasures in browsers (like SameSite cookies) will protect cross-origin POST and other state-changing methods, but will largely allow GET requests to go through while carrying session cookies.