Because this is an argument I have regularly, and it boils down to a wrestling fan weeping that the wrestler he liked lost and will likely be unduly harmed by such.
It's a stupid argument, and it deserves to be dismissed out of hand.
By the way, the "new talent" you want to push is a 40-year-old man who's had a very good career. He's not some up-and-comer fresh off the turnip truck. And I'm sure AJ Styles understands that a loss to Chris Jericho at Wrestlemania doesn't mean his children will go hungry for the next six months.
AJ may not be a spring chicken but in the 'WWE Universe' he is fresh and has the potential to be used in limitless ways. While wins and losses may not ultimately mean anything in a pre-determined show, it does still have an effect on how the audience perceives him and buys into him.
What if because a wrestler is handled badly when they arrive the audience doesnt get behind them or loses interest? WWE have done this plenty of times under the belief that 'wins and losses dont matter' yet they kill any interest the audience may have and 'burry' the star.
For years now people have complained that WWE doesnt push enough fresh talent and relies too much on established stars yet you argue that its fine for a new star to go through a programme with a legend who is only around to enhance other talent, having so many matches people are bored with the story only to 'finish' it off at Wrestlemania by having the new person lose? Yes they will probably continue the story but at this point does anyone really care?
To dismiss people's concerns as 'they are just salty their fave wrestler didnt win' is rather disingenuous and doesnt take into consideration WWE's history of doing things that are 'best for business' which actually ends up creating hatred or even worse apathy amongst the audience.