Sorry, I meant megastars created within WWE. The likes of Rousey and Fury brought in short term (Rousey had a good stretch to be fair) to bring eyes to the product and boost its appeal with the intent of retaining some of those eyes. Most wrestlers look like action stars but there's a difference between looking good and being able to act well enough for anyone outside wrestling to give a shit about you. The only three major wrestler-turned-actors I can think of are Rock, who is spectacularly charismatic and basically the whole package when it comes to entertainment. Batista, who is genuinely a very very good actor receiving acclaim for his range in a variety of roles. John Cena, still developing as an actor but is gaining roles beyond his early simplistic muscleman parts and looks to have a decent career ahead. Plenty of wrestlers both major and minor have had small roles in action movies but that's all irrelevant to the point at hand.
You've just said yourself that the fans forced Vince's hand in pushing Becky as far as he did. Even she was only promoted within the confines of standard WWE stuff - talk shows, a WWE movie, an ESPN magazine cover after the deal between the companies. This and his seeming unwillingness to capitalise on the surge of outward fame/popularity Sasha Banks gained from her Mandalorian role just further backs up the point that while he's happy enough to bring in outside stars for short term gains, he doesn't want to create a star from within that grows to exceed the WWE brand.
Question related to the bolded too - is part of the reason the WWE is far less popular now that they don't have any home grown superstars with appeal beyond wrestling?
Its a bit of A, B and C
A) Not many people get the big superstar push. Those who do were hand selected by Vince and often someone from the past such as Edge, Goldberg etc. On the rare occasion someone current does get the big superstar push. Drew would be an example of that. Its very rarely someone that the mass of fans chose. Those people sometimes get a push but very rarely the big superstar push like Drew, or Becky. So those are the few that stand out from the many that people might have accepted as huge stars but werent treated that way. Pretty much everyone gets a couple of weeks at some point where they're in a spot where they're doing okay and could take that momentum forward and do bigger things, but then after a few weeks they're just a guy again in the same spot as everyone else as Vince does something with someone else.
B) The booking. When you arent designated like Becky when Vince was on board, or Drew of last year then when the old guys return they are made a bigger deal of than you. They are the stars, you are lucky to be in an angle with them and put them over. Theres a huge holding pattern of people in this category. The guys who if they dissappeared one week and were never seen on WWE again, someone else would replace them and things would continue as normal like they were never there. These arent people teaming with a returning guy and gaining his respect and pushing on or something like that, they're just a body to beat and maybe put on a decent match so the older guy still looks like he has something.
and
C) The viewership. With twice as many people watching, someone who is really over with the live crowd and liked by the audience is going to see twice the appeal, be talked about more and have people outside of wrestling have a vague idea of who they are. The viewership would go up if there was someone with the talent and charisma of The Rock right now, but if the viewership was twice as much as now it would be easier for that same person to be seen as a big deal, so its a vicious cycle. A good example of this is AEW and the stars they've made. They do well in the ratings compared to the other parts of the show and competition and crowd interaction/feedback is very good, but its not like they're swelling the overall numbers. If Orange Cassidy and Darby Allin were seen by twice as many people it would be a lot easier for them to gain a little bit of mainstream appeal, but if they were truly special like The Rock then over time the number of people watching would go up, making it easier for them to be bigger stars.
I completely believe WWE would love to have someone like The Rock or Stone Cold Steve Austin. Can you imagine the financial offers they'd get from tv networks? They're already making all their money from this, lets say they have this star when it comes time for the next round of negotiations for their tv deal. As soon as the ink is dry on another lucrative 5 year deal, it doesnt matter if that guy then goes off to Hollywood. For the next 5 years they're making lots more money. And they could always try and bring them back as a part timer, they love to do that.
So I guess the point is for WWE's money making, having a huge star for the short term at the right time is what they need. Its keeping a huge star for a long time that would make more people watch longterm and become more invested overall. The 2nd part they dont seem that bothered by because of the first part being where the money is made.