A number of reasons.
Cost. A variable speed router is about £100. A home built rc motor spindle should be much cheaper.
1/8 inch collet. I can’t find anything other than a dremel that will accept a 1/8 bit over in the UK. Everything more powerfull is 1/4, and I don’t like the look of the adapters.
Noise. Rc brushless should be much quieter.
Repairable. If a bearing fails for example I can easily replace it. I suspect this will be harder with a router.
Speed. I can tailor the motor and or pully sizes to give almost any spindle Speed. Most routers iv seen are limited to a min of 10000 rpm.
Closed loop speed control. I might not ever need this, but i think it would be easy and cool to implement with this setup.
And last but not lease… I like making stuff!