As fun as it would be walking around, you have to ask, why?
You've kind of answered your own question. Because the very mechanic itself would be (for some players) fun.
And the bit in parenthesis is key.
ED is a uniquely mixed bag of gameplay elements. Cockpit shooter, space "simulator", strategy game, resource gatherer, driving game, arguably even a gambling game depending on your attitude towards Engineers. Some players embrace all of those activities, while for other people some mechanics are so onerous that they avoid them altogether. You might as well ask, "Why mining?", "Why exploration?", "Why SRVs" or even "Why combat?" You'll get a different answer depending on who you ask.
As an anecdote, I regularly park my explorer ship on an interesting bit of surface and use the camera suite and/or SRV to take a screenshot. The dullest "gameplay" imaginable to some players, I know, but I enjoy it and so do many others. If I could get my character out of the ship and walk up an escarpment to stare wistfully at the heavens like a cross between Matt Damon in
The Martian and Brian Cox, I would get a huge kick out of it. I might do it a couple of dozen times in a single session, or I might go weeks without doing it. But the
option to be able to do it is what would be important. The freedom. Any further interaction with the game that came along with it (what others would call "gameplay" even though to me the freedom would
be the gameplay) would be a bonus, but not essential unless FD locked an Engineer or some other game aspect behind it.
Now the counter to that, obviously, is that it's a lot of effort for the development team to go through if it's only going to benefit a subset of players with leanings towards simulation and a shared galactic wanderlust. And it's a fair point, and one that FD have sort of skirted around in the vaguest of terms without really addressing it. "We won't add spacelegs without valid gameplay," is a distillation of the argument. But as I've said, for some players the mechanic itself would
be the gameplay.
And this is where we come full circle to the problem FD has made for itself: some of those simulation-oriented players pledged at Kickstarter, or bought the game at launch, or bought a LEP, after FD
explicitly stated in both the KS FAQ and the Newsletter promoting the LEP that we would be able to leave our ships eventually. Some of the words used could be interpreted as self-limiting; "planet surfaces" in the Newsletter could mean surface stations only, so my Matt Damon fantasy might remain just that. But they have to do
something, or admit that they can't fulfil that goal. But they don't commit either way. They just continue to kick the idea so far into the long grass that it's almost over the horizon.
"Why?" is a perfectly good question, but it's a question that can be levelled at almost any existing feature of
ED so it's not the best argument against spacelegs. The unique question for spacelegs should be "Why not?" given that people have already effectively paid for it. And that's something only FD can answer.