The main reason is to create a Minimum Viable Product. This is the first step for any start-up.
After the hackathon, what projects should be doing is trying to secure funding. This involves the business side of things instead of the developer side of things.
Funding is usually secured by going to VCs, angel investors, etc and showing your Minimum Viable Product, pitch deck and so on. I think maybe 10% of crypto projects get VC funding and even less on TRON.
Projects can also join incubators or accelerators. There are many that are generalized crypto and not focused on one blockchain.
If you don’t want to sell out to VCs or join an incubator then you will have to make a profitable business/DAPP on day 1 to continue working on it. You will have to risk your own money for user acquisition and this is probably why most just give up.
And of course, the side product of this is developers just build to win the $$ instead of build business and become “Hackathon Hunters”