Because nobody can possibly know what a good requirement is, and what a reasonable amount of MN cap is.
We need a reliable mechanism that will help us find out the best price, because we are just humans. We are greedy and we are not infallible.

No human has the intellect to outwit the free market. Therefor we need to implement a free market mechanism to decide for us.

This is decentralization SQUARED!


Also, just so you are happy: if you want to run a secure SPR MN that nobody can challenge, then use this formula:

The amount of SPR deposit in your DM should be BIGGER THAN (All Available SPR) / (Max DM count) .

It is possible to run a secure SPR MN anytime, and the necessary amount will be easily predictable.