We never got any reported offers for Lingard, and he didn't seem to be in a hurry to leave either, he didn't push for a move. This makes a little sense if he wanted a squad role in what looked like could be a great season, and then having the power to sign anywhere come the summer if he didn't want to stay here further. Giving him a better sign-on deal in a hopefully better post-covid economic environment. One of his main motivations for going to West Ham on loan was also to get into the euros squad. With the World Cup in the winter next year, players can wait until the summer again before having to play frequently.
Henderson was from all reports intended to start as our #1 this year, but due to injury and then positive covid he had to start the season on the sideline. And since then De Gea has showed a real upturn in his form, making it hard to justify to start Henderson over him at this point. He was unlucky.
VdB's interest from Everton was only a loan, and they wouldn't even pay all his salary (Lingard to West Ham they covered all salary, plus loan fee, plus bonus for making Europa League). Plus he had only been at the club for one season so far, too early to "give up" on a player, even if he hasn't got a lot of chances yet this season either.
Mata was offered a squad role for less salary to be around the team as we didn't have that many senior players who had been at the club for a while. Less important what he does on the pitch, more what he does for the team on the training field and on travels. Plus they want him to move up to a club role (more than just ambassador) after he retires. It was an investment for squad harmony and the future.