in theory, if i had the prime conditions in a terrarium, why cant i just spread the spores from the syringe over the medium and water accordingly, and have them grow? ofcourse have everything sterilized.

if you can have an outdoor patch in your backyard from just spreading the spores over one area and having the right conditions...then could that work the same way except inside?

Well, you could and probably would get some growth. Problem is that indoor conditions are prime for indoor molds that will grow at a MUCH faster rate then your mushroom spores and contaminate your substrate and kill off your slower growing mushrooms.
Now, if you were talking of a sterile Lab environment, them the above is possible. But is hard to keep a area 100% sterile in a house.