Symbolic of JJ's approach to Star Wars
Same with Kylo rebuilding his broken helmet
Ehh Kylo's helmet is a metaphor for him being a punk ass wannabe Vader and unable to accept himself and other insecurities and needing to hide away. It's fitting that he would retreat back to that now that he's in charge, probably crippled by self doubt more than ever with that ethereal spanking he received at the end of TLJ by Luke.
Just slapping a bandaid on the busted lightsaber is pretty weak when it's destruction was the perfect setup for much more meaningful uses. A more meaningful direction would have been to salvage parts from the old lightsaber into the creation of a new one. Letting Rey make it her own. Not throwing it all away, but using those elements that still worked and were good. A bit of the old and a bit of the new. It would literally be manifesting the actual themes and messaging of TLJ into physical form.
Fallen Order did something very similar to pretty great effect.