Actually I honestly think GF's thought process on this is "if we're going to make a mobile phone version it's going to need some exclusive content to justify it". So they decided that while Switch's HOME would be how you transfer Pokemon over, the mobile phone version would be what you need to trade between HOME. They probably think this makes sense since you can already trade Pokemon in the game themselves... either ignorantly or purposely (or both) ignoring that Sword & Shield doesn't have GTS which makes trading easier. And that's not to mention the purpose of a mobile version is that it's for CONVENIENCE so doesn't need to have exclusive features. It just needs to provide the services we'd want on the go which it actually does... but it's the Switch's version which is the problem as it doesn't have the trading between HOME options which there was no reason for.
It's a stupid, out-of-touch way of thinking that provides no more value to the mobile version (if anything it takes away value from the Switch version) and adds additional layers of inconvenience to the players.
It could also have something to do with GF's paranoia with mobile phones, or rather mobile gaming, taking away customers (we'll just ignore Sword & Shield, despite the odds, are the best selling Switch games for 2019 and already one of the best selling games on the console; but no mobile gaming is such a threat...) that if they have anything mobile related they'd want to force people to have it on their phones. "HA! Now that trading between versions of HOME is exclusive to mobile phones they'll HAVE to download the app onto their phones!". You know, completely ignoring that most people would have done so anyway just for the convenience and they didn't need to chop that feature from the Switch version to do that.
Dialganet2 is right, this is JAIL!
This isn't home. home you're free to go in & leave whenever you like. Now sure some homes are bigger than others so only a certain number can permanently live there, but if you have more than the max amount of people that can be in your home all of them can still leave whenever they like.
On the flip side, jail you're sent to for the purpose of not being allowed to leave. In many cases you can only leave once someone has paid bail for you.
This is Pokemon JAIL.