It is done like this by intention, Instead of "healing" crashes by restarting FTL, we are much more interested in resolving the underlying issues such that a restart is never needed. Hence, we intentionally don't add such a button, because otherwise users would never (or at least much rarer) report issues which would make it much harder for us to find possible bugs.
Can you follow the debugging steps mentioned in the FTL README in order to assist us resolving the issue you are seeing? If FTL is running fine for you for some time, you can also use the simplified debugging instructions we provide here:
https://github.com/pi-hole/FTL#simplified-debugging-instructions-when-ftl-is-running
You can follow number 9 of the non-simplified instructions afterwards.