Because the show doesn't take place in the prime universe despite what people think?
Captains Log Supplemental: our attempt to return Mister Worf to his universe of origin has gone swimmingly. Whilst using the sensors to scan for quantum signatures and the fluctuations therein would be the more usual method, we have instead opted to ask him what uniform he usually wears. Combining comparative fashion analysis and my own studies of federation interior design, we were able to accurately define the correct universe to within one tenth of a moodyfan...the recognised standard for determining pan-universal differentiation according to Guinan, and she would know, she once dated Ted Danson apparently.
This has led to some distress from Counselor Troi, as it turns out that in the entire multiverse, only her husband was having her stitch labels with his name on into his underpants.
This reminded me of a Log from a previous enterprise, under the command of Captain Kirk, whereupon being informed of his uniqueness in the universe, and perhaps more by his CMO, he is recorded as having retorted 'we are ...like buses. There will...be another...in ten minutes or THREE at once.'
I have passed these findings onto Starfleet Command, and am informed they are already being implemented into the new Intrepid class interiors. It is hoped that by going for stylish and timeless designs, crewmen will simply not care or notice if they hop universes, or indeed turn out to some form of quantum duplicate, thereby saving hundreds of man hours in rescue attempts and artisanal carpentry. It is hoped this will reduce the sum damage to the universe by canonpedant particles by at least ten percent, and increase the success of temporal missions by at least one guardianogram.