Well, it's one thing to buy the script that powers the trading engine, it's another thing to take this product and transform it into a working business.
Running your own exchange is one of the most trickiest businesses out there and it is not everyone's caliber to run an exchange successfully. Yeah, a bit of knowledge and the techy person may be built up the exchange with the help of this script, but the script won't help you bring the customers and reputation with it.
A person who has a lot of money may do better by buying a running exchange rather than building up a new exchange from scratch. The most difficult part is to gain the trust of the depositor in times when people are afraid to keep the money even in the top exchanges.
The amount of work it would take a new exchange to thrive enough to get to the list of top exchanges in the world is insane. In the recent years, exchanges have set some standards that determines whether it's a shitty exchange or top-tier exchange. Exchanges like FTX make it even harder since there's less trust in centralized exchange services.
My stance still remains the same: Creating the exchange product - "script" is the easier part compared to actually getting customers or users to use that exchange so it needs to be a well thought out decision.