Why do people mine?
The obvious answer is: For various reasons.
What you really want to know though is this:
Industry/Crafting minded people that have a mind for business mine, because outside of initial investment to be able to do so, the minerals are free.
Now, since i run into it all the time when saying that…
Yes, most people especially multi-boxers pay $15 per month to play, so in that context they are not free, but then neither is anything then.
However for In-Game purposes only, ISK, the minerals are free past a certain point, buying the minerals from market can cost anywhere from 20%-60% of your profits base as a Manufacturer. Whether you are mining roids, or gun-mining.
As an exmple: take a ship using cruise missiles for missioning, t1 or t2 ammo?
You can bend over backwards making your own T2 ammo, or spend ludicrous amounts of ISK to buy the ammo.
Or you could buy hundreds or thousands of T1 at 1/10 the price.
Best option, buy a BPO, grind up your unwanted rat loot for minerals, build your T1 cruise missiles at the cost of only 4 ISK per missile, build more than you will ever need…sell the overstock @ 100 or more ISK on the market, just dont buy them because its not a smart move if you are using them as ammo. (hint using t1 ammo over t2 for missions will add maybe a total of 5 minutes to your rat killing time.)