That would increase the cost of the phone, since they would have to support and test the phone long past the expected lifetime. But, this is marketed as a budget phone, so they aren't going to do that.
It's unsustainable to put the costs on the backend, so phone's price would have to be raised to cover those extended security updates. But, that won't happen for a budget phone. There just won't be enough people willing to pay after three years.
That's the problem with purchasing a budget phone. You get what you pay for, a phone that will go away soon.