Others have already mentioned the issues stopping players from repositioning SPAAs, so I won’t go over those. I’ll just mention a point I haven’t seen mentioned here, which is the massive awareness gap between CAS and SPAAs at high tier (Top to ~10.3ish, when standoff range muntions become common).
CAS has a massive advantage in spotting and identifying threats. Not only do they get very high magnification optics, something which makes scanning for targets pretty easy (Doubly so for those that get thermals), but they also have third person view, giving them perfect spatial awareness of any incoming missiles due to the huge smoke trails. Said smoke trails can then be tracked back to the SPAA that launched it long before said missile ever gets close enough to threaten the plane, after which they can pretty trivially evade the missile and launch an attack against the offending SPAA.
Meanwhile, SPAAs get RADAR. Actually, some get RADAR. Some get a combo of search RADAR and IRST, meaning that while they get tracking indicators they cannot quickly or easily lock on to them. Some don’t even get a search RADAR, making them reliant on the good old Mk 1 eyeball to locate the tiny dots of aircraft at standoff range (Even worse if it’s a drone, which is practically invisible without thermals). And, as already mentioned, you cannot rely on RADAR, as doing so reveals your position to the enemy. Plus the search limits, meaning a plane that climbs high enough won’t appear on any scope except the Pantsir.
You ever wonder why you hear SPAAs doing aircraft pings at high tier? It’s because getting bearing and altitude information is actually more info than you get from the RADAR scopes of all SPAAs, and it’s all that SPAAs like the Type 81 and Strela get.
This is on top of the issue that planes have near infinitely more space to hide in than SPAAs do. As already mentioned above, maps are tiny for the purposes of top tier, and even smaller when it comes to places you can hide defenseless buses like most top tier SAMs. Meanwhile planes can come from practically any angle and altitude they wish for, meaning that while SPAAs have to watch the entire sky in case someone’s trying to flank them, CAS merely has to scan small sections of a tiny map from safety.
Finally, the real nail in the coffin that I’ve not seen mentioned here, that idealized scenario you have in the OP relies on the SPAA player having time after spawning to move into cover, something that’s in no way guaranteed unless you first spawn SPAA. Often times, by the time you’re spawning SPAA there are already CAS aircraft in range to engage you the second you spawn in, and the best way to help them do that is to slowly trundle your way to cover before engaging them. Even if you make it, they now have you pinned in cover and can make a pop up/vertical attack while you hope they get sloppy enough to forget you exist and eat a missile.
It also doesn’t account for multiple aircraft. Admittedly, this is already a lopsided fight, but it’s not an uncommon one, and unlike at low tiers it’s a lot harder to quickly switch targets as they make attack runs on you. The entire time you’re guiding a missile in at one plane, every other plane knows exactly where you are, and you can’t move without throwing off the guidance.