In theory that's a sound assumption, but the problem is that all the fiddly bits that control the rate and damping and the adjustment of that, are also in that open bath. So when you start to open them up, there are many more parts to have to take account of and put back together.
To illustrate the point, I once tried to change the spring on a pair of 888s. Very soon after having opened them up, I realised I was way out of my depth. One of the bike shops local to me (though not my local) agreed to help me out and made a right pigs ear of the job. I ended up crashing really badly (I hit the wooden parapet of the bridge at the bottom of the Cwmcarn DH track – the one you go under at full tilt). Oil had burst out of the top of the stanchion so I sent them back to windwave who then informed me that nothing had been put back correctly and there was 30% too much oil inside.
So if a bike shop can make that much of a mess of things, it's clearly not a simple job.
With sealed units, you're only ever servicing the inside of the stanchions, i.e. new lubrication oil between the bushes etc, rather than the actual damping oil.