Even on sales, this game is ridiculously expensive. Why?

Probably because they're still actively working on it.

But, judging by the reviews, it is rather mediocre game. I am somewhat interested because I like the theme, I like science fiction, starships, new planets etc.
While this price, it would fit AAA+ game, and very good one at that.
When compared with Planet Nomads, it is absurd.

IMO, it's more than a mediocre game. Make sure the reviews you're reading are talking about the updated version. The release was extremely rough and the the devs and game have taken a big reputation hit because of that. But they've been constantly updating the game and adding new features. It's a far different beast than it was at launch.

As far as the price goes, it is what it is. If you don't think it's worth the price, then wait for a sale. If you still don't think it's worth the price, then don't buy it. I got it on sale in the summer of 2017 and paid around half price but never really played it until last year after NEXT dropped. I've played nothing else since then, clocking over 1500 hours in this thing. In my estimation, it would have been more than worth paying full price.

This game worth every penny! Just make sure this game is for you. Check gameplay videos to have a better idea.

I'm having a blast with archaeology alone. And that's just a tiny part of the game.

Devs keep updating and bring so much to the game with every update. Other devs would turn those updates into DLC instead. So yeah, wonderful devs and wonderful game, and totally worth the money!

Really? In the sales I got if for less than half price which was pretty good inmho.

Ditto--HG has continued to add value to this game for years--and free of charge, too! It's a great value if you get it for half off--I did.

Huge content updates with another one coming on August 14th, all free. If that doesn't justify a higher price, then as I said before, don't buy it. I feel the game is actually well worth full price. I've played nothing else for an entire year at this point.

While 40GBP might seem a bit high for 3 years old game on PC the 50% discount makes it quite accessable.
If you like games like Elite Dangerous then you should try No Man's Sky for sure.
And like others said, the amount of free udates that Hello Games pushed out for this game and NMS 2.0 coming out in a day or two definitely justifies the 20GBP when bought on sale.