There are two things to cover here - firstly, the point of your original question, but also the bigger question of ‘why don’t banks incentivise existing customers to be loyal’.
Each bank has different policies when it comes to rewarding points to ‘new’ customers. Amex is 12 months, as you could see. Others have lesser requirements, or have no written policy but will look to see if you are applying for new cards with them a lot and decline your application if so.
As a result, signup bonuses are useful for different people at different times. There are people who know all about points and looking for a new card to switch banks from, vs those who are new to the whole thing and looking for their first. Travel rewards cards are fairly mainstream, but not everyone has one, and banks are always looking for new customers to bring on.
To the bigger question of why… that’s a tricky one, and not one I fully understand either. I think banks should incentivise loyalty more, but I think mostly it’s down to the fact that marketing budgets are all about new customer acquisition, while loyalty and retention are secondary success factors. Banks do look for the ‘right’ customer, not just any customer, but don’t factor in loyalty enough into their marketing, in my opinion.