they didnt do that because people would be constantly complaining about quests being broken and dragons can attack basically anywhere so important NPCs can randomly die, just look at Oblivion if you wanted master archery training you had to do it before you were like 10 hours into the game because she would constantly die
That was a case of not paying attention when they selected her campsite, she should
never have been put there in the first place.
I suppose they should have put all the trainers in
Skyrim indoors, since Dragons can't reach them there, however the ones outdoors are still a lot better protected than the outdoor ones in
Oblivion, and actually have a decent chance of not getting roasted.
The quest aspect is really a no-win scenario for Bethesda; the current setup pisses off a lot of people (myself included) because it makes us feel like we're being treated like children, rather than advlts, but unfortunately there's a large number of people who, for whatever reason(s), simply cannot grasp the concept of 'consequences' and get all steamed when a quest breaks after they killed a related NPC. Unfortunately for those who feel as I do, Bethesda chose to cater to the 'I can't handle failure!' section of the fan-base in this regard.
To show you how silly it gets, I'll borrow a previous poster's example of raiding an Imperial fort: when you do this quest you discover (as he mentioned) that the
Imperial Generals are essential, which is just [censored] since the
entire point of the quest is to drive them out so the Stormcloaks can take the stronghold for their own use. The
only time these NPCs should be essential is if you're backing the Empire, since then you want them to live.