Low recoil and noise mean no flinch. Most have smaller bolt handles and easier controls so anybody can be taught to use one.
It's not the one you have to start with. The first gun I shot was a .380, then a .40, then my 9mm. It wasn't until after I bought my handgun that I bought a .22 rifle.
Between my handgun and bolt action rifle, there's only one I can shoot all day for $20. I'll give you a hint: it's not the handgun.
Other rounds like .17 HMR, .22 WMR, etc. are all fine rounds, however, the .22LR has even more advantages over those.
More readily available
Cheaper
More selection in rifles and accessories.
You could start with a .223 or a .243, but at one point those were both a dollar a round. You could start with a .308 (that was the first rifle I shot). We're only able to shoot ours about one out of every 3 range trips, maybe up to 40 shots between the 3 of us, because the rounds are so expensive. Not to mention the recoil and flinch.
Many fine shooters have started with a Mosin Nagant and a spam can of Russian corrosive ammo.
But I think most here who have owned a .22 will agree that it's been the most effective form of training because it's cheap enough to shoot a lot and the low recoil means hits and misses are a result of your technique which you can easily work on when not fighting the kick of the gun.
I still don't have a .22 handgun. If the Kadet kit for my CZ was cheaper than $400, I'd have one of those in a heartbeat. It costs me about $70 to go out to the range and fire a couple boxes of ammo right now. As a result, I tend to lose my recoil impulse control and other progress I make because I just can't afford to shoot it enough.