You want to protect your bikes from:
A tent offer no protection from thieves, but let's assume it is not an issue.
Not all elements are equal.
A bike can stand UV (the sun) quite long, let's say up to 5 years (especially counting are summers). A tent will not stand that many years (i.e. summers) of UV rays.
Unless you are living close to the sea, where the air will be charged with salt, you need no protection from wind (ok, wind may carry rain, but then the issue is the rain). A tent would provide superior protection to wind, with respect to a tarp.
Very important to protect your bikes from the rain, but a tent would do a job as good as a tarp or a cheap DIY shed
This is not the same as rain, humidity makes disaster on rubber and plastic parts, as well as accelerating rusting (usually not a strucutral issue, just an ugly patina ... ugly minus the years when such a patina is cool). You avoid humidity when having enough ventilation. A tent is very bad at this.
Snow can do damages as providing water that infiltrates and then freezes (if in your location there is snow, there is cold and there are freezing conditions), breaking many things and displacing any grease you have in bottom brackets as well as in the wheel hubs. And derailleur and gears, poor guys!
Tents may collapse and break under minimal snow weight.
WILDLIFE (here only for the sake of brevity, apologize for including them into the elements category...)
Ranging from mice to spiders to killer bees, a tent provide an ideal warm and humid shelter to many companions ... unless you enjoy taking out your bike and a wasp nest, they are not the companions you want for a bike ride start. A tent therefore is not a good idea. This apply as well as to a tarp, although it is way easier to clean it, as well to a shed, but it is easier to place traps and other wildlife repellers in a shed (all these repellers suffers from humidity, while wildilfe generally not, so see point above).
Final point: you can easily store 4 bikes in the tent, but how easy is to take a particular bike out of the tent?
This question applies to tarp and to a shed as well, but you can be a bit "rough" with a shed or a tarp, while a tent is easy to be cut/rip, so after a couple of rushed bike extraction maybe your tent is not as effective as it was.