No, it's because they've all achieved more than he has while being younger to boot.
Keita is 23, has played in the Champions League and dominated Bundesliga games against top-quality opposition - Fabinho is 24, and has won Ligue 1 with AS Monaco. Alisson is 25, has just transferred for a world-record fee after a season where lots and lots of people considered him one of the world's best keepers, up there with his compatriot Ederson.
Meanwhile, we are discussing spending the same amount on a 25-year-old winger who couldn't cut it at United and has spent seven seasons in total at Crystal Palace, registering mediocre goals and assists tallies in both the Championship and the Premier League. And this despite being afforded a lot more space and time than he would in a more competitive, high-end side.
It hasn't always been these Carlos Kickaballs that have turned out awful - we have many, many instances of signing awful players from our own shores as well, as I'm sure you're well aware.
While I can appreciate the points you make I think that there is also a decent counter argument against all of them. For example you say that Keita has dominated Bundesliga games against top quality opposition, but Zaha has also done this, except he has done it in PL games against top quality opposition, which considering we are in the PL, surely trumps doing it in Germany? I'm also not sure that a player winning the league in France cuts too much mustard really, after all we signed Aurier who had won multiple titles in France. It doesn't automatically make him a great player or one worth a fortune.
I also think it is a little unfair to judge Zaha on his time at Man Utd. He moved there at only 21 years of age, before he was anywhere near fully developed as a player and ready to do so. Additionally the manager who signed him then left the club 6 months later. I think the move to a big club probably came 3 years early for Zaha. He needed to develop further, specifically to improve his decision making and his end product before being good enough to play at the top level. I think he has now improved both of those things and is ready for that big move.
I completely disagree that Zaha would be afforded less time and space playing for us than he gets when playing for Palace. Zaha gets more attention from the opposition than any other player in the Palace team and, from what I have seen he gets more attention than pretty much any other player in the league. Teams that play Palace often change their shape and tactics just to try to counter his threat. I think he would get
more time and space playing for us because teams cannot afford to double up on him at the expense of leaving the likes of Kane, Eriksen or Ali free. Additionally being surrounded by better footballers will result in him receiving better passes in better areas.
When I watch Zaha I see a player who has all the right attributes to play for a club like ours. Pace, power, skill, vision, bravery and confidence. The only concern I had about him previously was that he seemed to happy to stay at Palace and be a big fish in a small pond, I was therefore worried about his will to win. However if it is true that he has decided to reject their new contract offer and ask for a move then that is the final box ticked for me.
In terms of what Zaha is worth, that is a difficult one. As Palace's best player and one who they would almost certainly have been relegated without last season and the player still on a long contract, clearly there would have to be a premium for Palace to agree a sale. I would say that as a player in a decent foreign league Zaha would probably cost about £40 million, you then have the gamble on whether or not they will be able to adapt to English football. Add a £20 million premium for him being PL proven and homegrown and you're up to £60 million. On top of that though would have to be the premium for Crystal Palace and that probably takes the fee up to £70 million. Is he worth that as a player? Probably not, but would it be worth us signing him for that sort of amount?.... Perhaps it would be. He is clearly a good player who can play as a wide forward or through the middle, is the right age for a move to a club like ours and still seems to be improving.