I don't develop for Macs or iOS, but I can imagine a few reasons why most devs don't want to do that.
apple has made it so easy for them to convert their apps for the M1 macs
It may be easy to build an iOS/iPadOS app for macOS, but the result will always be an app built for a different form factor on a device where it doesn't belong. Touch-based interfaces and mouse+keyboard interfaces work differently and a 1:1 port from one of the mobile OSes will NOT feel like a Mac app. There's a lot of UX work to be done on a Mac port of a mobile app and it's just not very profitable:
iOS apps target all iPhone users.
Mac ports of iOS apps target only the tiny "people who are okay with running ports of iOS apps" subset of the tiny "Apple Silicon Mac owners" subset of all Mac owners.
apple should make it mandatory if you have a iOS, iPadOS app it has to be compatible with the M1 Mac App Store
Apple requires apps on their platform to conform to a certain standard, there are interface guidelines to follow. By making it mandatory to support macOS they would open the floodgates of terrible one-click 1:1 ports that would literally ruin the platform known for expensive "boutique" apps.
Ask Microsoft how it goes with UWP apps. And when you're at it, ask ChromeOS how's Android apps support.
Running iOS or iPad OS applications on macOS shouldn't be a top priority. In fact, it shouldn't be any priority at all.
This is only my opinion, but I have to say I 100% agree with this. iPhone and iPad apps optimized for touch are not welcome on my fairly expensive work tool with a 43" screen. It goes against my interest that Apple pours any dev time and money into this. I wish they didn't do that.