I think Canon wants first to show off and make clear that they are very serious with their R system. As others here I am pretty confident that we'll see soon more lighter, smaller lenses (zooms) that aren't that fast for the RF mount. The RP is already a hint that Canon makes a move into a more consumer oriented market segment.Probably for the same reason I would.
While impressive there's very few lightweight RF lenses. In fact so far there's the 35mm 1.8 and just a 24-240 on the horizon.
Granted, the RF24-105 is cheap and suitable for a lot.
Btw with heavy lenses you always should keep in mind, that Canon helps you to train your physis - for free . When I take e.g. my heavy EF 500mm lens out for birding I regard it always as a photography-sports combo. It is capable of delivering beautiful images while using it freehand as a sort of dumbbell - a classic dumbbell can't do that, so it is really dumb .