I usually buy ready made pizza base and use it for baking pizzas. All of my pizzas get a very hard and crispy outer crust. Why so?
I thought of brushing some oil on the pizza base before I bake it but no help. I use a convection microwave oven.
The way you describe it, it sounds as if your oven uses microwave radiation while you are baking the pizza. This is obviously a bad idea, but a manufacturer could just add "pizza setting" as a selling point, without making a reasonable implementation. Or maybe the oven is OK, but the user interface is confusing and you somehow don't switch on the right setting.
First, try to find out if the oven is supposed to turn off the radiation while baking pizza. If this isn't described clearly in the users' manual, call technical support and ask them. If it is not supposed to turn it off, your last chance is that the oven has the possibility to manually override the setting and turn off the radiation. Else, this oven is not suited for pizzas or any other kind of dough.
If the oven is supposed to turn the radiation off, and you are sure you have selected all the correct options for it to do so, but your crust is still too hard, then the oven is probably defective and radiates microwaves when it shouldn't. Sadly, I don't know any way to test for that (theoretically, you could put a small piece of metal in it and look for sparks, but you risk damaging the oven that way.). Maybe if you have a friend with access to a physics or a sepcialized photography lab, they could find you some microwave sensitive film, but the probability is slim.
If you somehow find out that the microwave definitely isn't nuking the pizza, the only other problem I can think of is that it probably gets too dry when baking. You could try baking a thicker pizza (put two pure bases on each other and roll them a bit with the rolling pin, they should combine) and/or adding more sauce and fluid toppings/cheese. Or reduce the baking time. But it would take a lot of drying to make the crust unpleasantly hard, so I am 90% sure your problem is caused by microwaves.