Because it costs money to build a machine, rent the space, obtain electricity, send a human to collect the coins and then pay the bank fees to deposit them (presumably a business account).

HSBC headquarters basement and Mong Kok basement charges 2% for effectively unlimited amounts.

Alternatively there are the HKMA coin carts but you have to get to the place where they are that week.

You can also just count up $50 of currency and top up octopus with a human at an MTR station or convenience store or McDonald's