In Star Trek it seems that they almost always have artificial gravity. The idea of not having it is shocking, even though my 21st century mind relates it directly with space. Here are a few examples:
Deep Space 9 episode Melora, Dr. Bashir shows wonder and amazement at being in a low gravity environment. (He'd really never been outside artificial gravity?)
In the Star Trek VI, when the Klingon's gravity is disabled, they even refer to themselves as being helpless. (They never trained for this?)
In the TNG episode Ethics, Worf gets severely injured because something like a barrel, fell on him in the cargo-bay. (Couldn't the gravity be reduced in the cargo bay at least?)
In the TNG episode Hollow Pursuits, Barclay has problems with his anti-gravity device. (Why not just switch the artificial gravity off?)
The only explanation I can think is that for some reason artificial gravity is required for space travel, but is there any canonical explanation for why they cling so doggedly to gravity?
I'm looking for an in universe explanation. I'm also curious why the gravity generators are rarely targeted, but that may be another question.