in short, BG has trash, I don't think they have any tech for multiple spawn points in an instance.
check out the RoI 3mans, each one has a variation of a work around of the issue;
- Fangorn's Edge
after killing all the trash, the train of adds that go up and down the ramp stop allowing players to use it as a shortcut.
- Dargnakh Unleashed
there a mario like piped that unlock after killing the trash that teleport you close to the bosses.
- Pits of Isenguard
the map isn't very linear, so after clearing all the adds the players can bypass much landmass and speed to bosses much quicker. this method is probably more noticeable in instance like Urugarth or Carn dum, you can essentially forge your own path depending on what bosses and quests you want to complete.
you can see by design they try and make every raid a slightly differant layout, and that layout depends on what the designers want you to feel. for example BG is long road up the tower, and finally you get outside. that constantly feeling of going up is does give you some natural dread of heights and fearing whats at top.
Lair bosses still need context
when you compare that to lair bosses like turtle, watcher or any erebor instance, the designers wanted the focul point on how hard the fight was anyway. those tiny bits walking in might seem useless, but it's NEEDED to put into context what your about to fight (like the highs and lows of gameplay, we as humans love highs more when he have lows).
lastly death... death needs a punishment, whether it's an annoying walk to the top, or massive repair bill. when there is no punishment of death, death becomes almost common place and it becomes more of a "hahaa, you failed again" instead of "try again?". there should be a kind of corrilation of how much they expect you to die while doing this, if they know you die lots, they put the spawn really close so you don't go mad. if the boss has a 50/50 chance of completion, they'll put a huge walk so your promoted to get it right first time.
Complexity vs Depth
LT wasn't that complex, it had ALOT of depth in the fight of what to do, but down to figures it was pritty easy when you look at damage taken from boss, and healing out-put (hence why they added in power regain debuff so you didn't spam, it was all down to tactics)
however, look at BfE, lodes of complexity and lodes of depth. your dps, tanks and healers need to be at top tier while following the tactics precisely.
if your confused with the last point, I can link a video but I don't want to derail too much. it's all down to design though