Why? Because Rollup.js makes it possible to structure code into modules in a way that is backwards compatible and future proof. Yeah it takes some getting used to and it is not yet as simple as it could be, because integrating libs which are not yet available in the module format, takes some manual configuration, but once you know how to use it you won't want to got back.
I can highly recommend it, not because it is the hottest tool out there, but because I think it has a much higher long-term value than most JS libs.
Parcel seems to have a similar functionality but due to the wide range of other functions it comes with, I think there is a much higher chance that it will be rendered obsolete in the near future. Rollup.js on the other hand _focuses_ on the combination of modules to a single bundle which will continue to be required for the foreseeable future.