Because creating a new product is harder and more expensive than changing an existing one, even if it does destroy it.
Advertising, novel technical infrastructure, new branding, etc... all of this needs to be done for a new brand -- and then who knows if any one will come visit?
Or, you can just make whatever short-term changes to your existing successful product, juice the metric you're looking to juice, and (in theory) cashout before the long term repercussions take affect.
New branding isn't necessarily bad; it can make a new product seem fresh. Technical infrastructure, both hardware and software, can be reused. You don't have destroy one software product to make another.
Take this OkCupid example; they had their website model. They could have just created "OkCupid Nights" as a separate Tinder-like product reusing as much of their tech as possible.
Advertising, novel technical infrastructure, new branding, etc... all of this needs to be done for a new brand -- and then who knows if any one will come visit?
Or, you can just make whatever short-term changes to your existing successful product, juice the metric you're looking to juice, and (in theory) cashout before the long term repercussions take affect.