I agree that 18-45mm f/sluggish is a fairly crappy spec for a package lens. I believe Canon took a budget and straightforward path for it. Evaluate the EF-M 18-45 (left) to the RF-S 18-45:
The RF-S model is an analogous design, utilizing smaller components however missing the rear diverging correction group. Not like the RF-S 18-150 which merely used the EF-M 18-150 optics in a brand new barrel, that might not be accomplished with the EF-M 15-45 design as a result of the 2mm longer flange distance of the RF mount means a 15mm lens must be a retrofocal design (just like the RF 16/2.8), and that might have resulted in the next value than Canon wished.