Its a side effect of an optimization function, basically. New customers are price sensitive. Loyal customers are.. loyal. They will buy anyway. Advertising is expensive, and discounts can be a cheaper way of getting them. If your decision framework is "do what brings the most growth by lunchtime" you will often end up with new customer perks.
If you have freebies and discounts to spread around, spending them on newbies will produce more sales.
Politcal campaigns often consider voters in reverse order to loyalty. Party members are guaranteed votes, so don't spend effort courting them. Party supporters need some attention, to bring them out. Fence sitters or potential conversions are courted vigorously.
Lots of tech "platforms" will be free for new users, cheap for small users and expensive for big clients. The big clients are more locked in, so it's hard to affect sales one way or another. New/small users are fickle, and more likely to respond to prices.
Banks have better rates for new customers, sometimes actual free money.
Its like when you don't put effort into dating your wife. It's not the right thing to do, but there are some difficult to avoid consequences to knowing she will go home with you regardless.