To be fair, since the minimum rent time of these instances in the cloud is 24h, chances are even a cloud-provided instance is unused for quite some time.
Unfortunately Mac instances have a minimum billing duration of 24 hours, so even if you only use it 8am-5pm, you'd be billed for 8am-8am. (Not AWS's fault, it's part of the Apple EULA.)
Money. Disabling a Mac for 24 hours after 10 minutes of use means you have to buy much more hardware than you'd have to if a single Mac could be utilized by many people for short periods of time. Some users would use a single Mac for days/weeks/months but others will want to boot, run a build, and be done. The actual work might take 5 minutes or 3 hours but you pay for a day no matter what.
Days where you're doing the same short-lived task many times won't be as expensive per build, but if you just want to do a nightly build, you're paying for months or years and utilizing a fraction of the compute time you're buying.
Probably they are deliberately trying to make M2 EC2 instances expensive to avoid all their Mac Minis being bought out by AWS, they get higher profit margins selling to retail customers.