The field theoretical treatment of gravity..or rather...quantisation of the gravitational field..leads to GTR. Thats awesome. You can also describe electrodynamics with qft, ok...qed rather....you can have a flavourspace using non abelian gauge theories...so why in the world discover string theory?
Problems with the concept of a string:
Now, the smallest entity of everything is usually imagined to be a sphere...simply beacuase it is the most symmetric shape in 2+ dimentions. Also you can neglect a lot of complications if you just consider a sphere. Rather...a small sphere of negligible radius. For example...you dont have to consider any self energy ...or any energy density for that matter....but a string being an extended object has all this. So maybe that helps you satisfy Einstein equation where tension T (or rather the energy momentum tensor) has some significance, but couldnt you just do it all by qft. qft gives me the feeling that it is the science of all...it can explain any interaction.
