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Because very occasionally the mirror needs to be cleaned. To do that it needs to be removed from the tube and mount. after cleaning and remounting into the tube , it then needs to be recollimated because no mounting system will perfectly replicate the previous collimated position.
Technically it would be possible to design a perfect system but that would require added complexity that would add , maybe substantially, to the overall cost of the scope.
There is no big drama in collimating newts, maybe the first few times you have to but unless something serious has happened to the alignment, it only takes 5 minutes with a cheshire and then laser collimator to tweak the alignment. I regularly take my newt / dobs for 100 km round trips to our dark sky observing site in the back of my van and when checking collimation on set up, only takes a minor tweak of the screws to get it all aligned nicely again.
Unless you are going to have a permanently mounted scope that will not be subject to vibration or temperature changes then having a flexible system is much more convenient for us amateurs and the cheapest and simplest to design.
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