Yes, this has been requested numerous times before. The best guess is that the reason it works the way it does is to minimize the risk of corrupting your data.
As far as I know, RM only has one competitor which has the ability to share data directly with Ancestry including sources and media, and that competitor does operate in an automatic sync mode. Users of that competitor occasionally report that their database has suffered significant corruption as a result of the automatic sync process. And indeed, there are users who have switched from that competitor to RM after suffering a corruption event.
Just from watching the RM forum here and on Facebook, it seems to me that RM users are in one of two camps with respect to how difficult TreeShare’s update process is. One camp seems to report that TreeShare’s update process is no big deal. They simply do a TreeShare update on a regular basis, such as daily, and all is well. The other camp seems to report that TreeShare’s update process is extremely onerous and wish for an easier way.
I’m in the latter camp. I find TreeShare’s update process to be so onerous that I never use it. Instead, I occasionally delete my tree from Ancestry and upload it again from RM. That’s very easy to do. The only disadvantage is that I lose the status of my Ancestry Web hints. Similar to the two camps about how onerous TreeShare’s update process really is, there seem to be two camps about how critical it is to maintain the status of your Ancestry Web hints.
There are a wide variety of ways that RM users use TreeShare. The main three categories are to work only in RM and to update Ancestry from RM, to work only in Ancestry and to update RM from Ancestry, and to work in both and to update in both directions. Opinions about how TreeShare might be improved in the future seem to vary depending on which way you use it.