why do criminals get bail?

Trials can take a long time to schedule. In some cases, it could be as long as two years before a case comes to trial. IF the accuse is really dangerous, or if they are a flight risk, they are not supposed to be allowed bail.

If the judge is convinced by the defense that the accused is NOT dangerous, and is NOT a flight risk, then the accused or his or her family may pay bail, so that the accused doesn't have to await trial in prison. Sometimes they are placed under restrictions, such as an ankle monitor.

They don't. People who are awaiting trial for criminal offences may be granted bail, but never a convicted criminal.

what you are actually saying is the accused are already guilty, until proven innocent at trial.

Because only 10% of federal cases and 25% of state/local criminal cases end up not going to trials.