I thought the same thing, which sent me down a bit of an unexpected rabbit hole in the topic. Greg Paul argued that thr chevron shape of the bones in the bottom of the tail point to sauropods rearing and using their tails as support . Heinrich Mallison did some biomechanical modeling and found that some of the anatomical features previously thought to support rearing might actually hinder it. And last year, a study on larger sauropods (Dreadnoughtus and Giraffatitan) showed that their femurs most likely couldn’t handle sustained stress of resting.

So it looks like this pose is based on anatomy, not biomechanics, and the one rigorous biomechanical sauropod-rearing study that exists didn’t even test this genus - which means the rearing question Mamenchisaurus is unresolved.

https://en.wikipedia.org/wiki/Mamenchisaurus

https://reptilis.net/DML/2009Apr/msg00036.html

https://onlinelibrary.wiley.com/doi/10.1111/pala.70019

ALSO, consider how stiff their neck was, it could very well have spent most of the time grazing on the ground, like you said!

That’s an ongoing debate within paleontology that often takes place on a species by species basis. The argument goes that since diplodocids (which this species is not) had heavy muscular tails, their center of gravity was near their hips, making it easy to rear up since they were effectively already a balanced seesaw and could use the tail as a third point of contact to balance. Species in Mamenchisaurus share similar pelvic and tail features and M. youngi was show to have a stiff neck that couldn’t lift very easily, so it’s inferred that these may have reared as well. There are center of mass and skeletal models and stuff to determine whether rearing is possible but one hasn’t been made for this species specifically.

Sidenote: you underestimate the cardiovascular cost of pumping blood up a 5-15 meter neck. It’s not at all clear that a rearing strategy is more expensive energy wise. In their case it’s less spending energy to standup than just leaning back to let their skeletal structure and center of mass do the work.

Mostly I think this pose is a matter of logistics. They probably just had more vertical space than horizontal to work with for this exhibit. Even though they’re fiberglass, the casts for these guys run well into the tons per skeleton so it can be challenging to mount the armatures in an existing structure and it turns into a game of fossil tetris balanced by the cost of structural support modifications needed (there almost always are for a fossil of this size).

Apparently there's a population of African elephants which rear up and balance to feed higher.[1]

[1] nature video starts with example: https://www.youtube.com/watch?v=3XzQ4BQe4fM short clip: https://www.reddit.com/r/NatureIsFuckingLit/comments/19bge4y... longer clip with two: https://www.youtube.com/watch?v=Jpxgqu_Cfkg

If they could eat from higher branches than others, they could avoid competition for resources.