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 ...
Another reason tents are a bad idea for long term storage is that very few will last long in the sun. Canvas is better than synthetic, but would cost more than a shed.
I've had caravan awnings up for months on end, and use groundsheets to cover garden toys, and all go brittle eventually, then tear and fall to bits. Even in my rather cool, damp climate, one ...
The main difference is that the tent gives almost no protection against thieves, where a shed gives a little more protection as it is.
And a shed can be improved a lot with a simple anchor ring which you fix to the ground or through a wall so it is very hard to remove, and to which you lock your bikes.
A well set up tent of a decent quality will keep the ...