Paid user acquisition is spending money, usually on ads, to get people to install your app.
The app business is now a ruthless winner-takes-all market and the game is all about spending $3.00 to make $3.05.
When you see pervasive ads for Mobile Strike or Game of War it means that these games are doing a better job of monetization than anyone else so they can buy the users and dominate the top spots in the charts.
Anyway, to advertise for your app, you're competing with every other advertiser out there with products that earn at least a couple dollars per clicking user. If your app only earns $.15 over the lifetime of the person that clicked, you can't afford to take out ads.
As to "Why can't you change it now?" - I haven't figured out a design that would ever allow it to be a high LTV product. So what I do going forward is think very carefully about potential apps through the lens of whether or not they could possibly be high lifetime value. If not, I won't waste my time writing that app.
In closing it is a really really damn hard craft to master, but the multi-dimensional aspect of it keeps it fun.