why can't I (or can I) ask for a deconfliction service in VMC and practice my instrument flying without a safety pilot
Because the controller will not be able to see anything made of plastic.
Even on a procedural service into Cranfield last year, a glider shot across the hold about 200 feet above us (I was lookout for someone under the hood).
As soon as the real weather comes in, you're actually much safer. I read/heard that there hasn't been a mid air collision in IMC since WW2. Someone is sure to bring up a more modern statistic but the truth is, real IMC is usually devoid of aircraft that are not under some kind of service.
The opposite is true on a nice VMC day.