r.e. dumb question
It's a fair point but partially it's to do with chemistry - life is a series of chmical reactions and the physics required for biological reactions favours certain elements more than others. That's not to say other possibilities aren't out there though.
The big problem with the 'there's loads of stars so there must be loads of Earths' theory is taht it ignores several low-probability things that have effected how the Earth develop. Firstly, we were hit by a mars-sized object in our early history which ahs provided us with a truly massive moon for our size. Due to its size the moon has been able to stabilise a lot of the orbital eccentricities that effect other planets. These eccentricities would have lead to extreme climate variation which would in turn have made it very difficult for life to get a foothold.
We also exist in a fairly narrow band of space where water is neither frozen all year or boiled off into the atmosphere.
On top of that, we have a liquid iron outer core and solid inner core. The geomagnetism produced by this gives us a protective shield which reduces the ability of the solar wind to a) irradiate us all and b) strip away our atmosphere.
There's a whole host of other peculiarities. The other thing people forget is that there is an observer bias here. It may seem odd that there are no other planets like ours, but if very special conditions are required to get life then life can only evolve to observe those conditions. As a result you then take those conditions for granted and assume they must be common.