They need to keep their "bubble" on the same growth level. The idea is very simple, the moment the bubble "indicates" that is drying out, investors will withdraw money from it, further reducing the profits for others, therefore others will also withdraw. Hence if no "profits" are generated "artificially" via depth creation, there will be no REAL NEW investment.
This is why the normal monetary system is doomed to blow one day. Crypto will take over, I am sure. When is (was always to my view) the question...
Late thank you for this excellent comment @forykw
Yours, Piotr