I was thinking the same thing as the earlier posters, but being curious I decided to check.
What I found is that actually, the latest iPad (8th generation) appears to ship with a USB-C power adaptor, 20w, along with a Lightning to USB-C Cable, see:
View all technical specifications for iPad (9th generation).
www.apple.com
It looks like that power adaptor cannot be used with the 5th generation iPad, see:
The Apple 20W USB-C Power Adapter offers fast, efficient charging for the new iPad Pro models. Use it at home, in the office, or on the go.
www.apple.com
Expand the 'Compatibility' section and it will give you a complete list of Apple products that work with the charger.
Previously, base model iPads came with the lower power 12w adaptor, which is compatible with 5th gen:
Apple 12W USB Power Adapter lets you charge your iPad with Retina Display through an electrical outlet. Get fast, free shipping when you shop online.
www.apple.com
I'm not completely convinced that the USB-C adaptor can't be used with the earlier iPad models, with the appropriate cable, because I thought these devices always managed the recharging rate to deal with chargers of various specifications, but you can see for yourself what the Apple spec pages say.
Perhaps someone else who is more knowledgeable can help us understand why this is.