it is amazing how fast you use up those billions of calculations per second. especially if you looking at the "calculations" it can do. a better term would be "operations". these calc/oper are very simple. a normal computer doesnt solve billions of calculations you do in calculus class, they do billions of simple things like: a+b. but even that you first have to store something in a and b. so first you say put 1 in a, thats one operation.
to put some light here is a high level or moving your pointer on the screen. now think about each of those "simple" things and break them down to stupid simple.
ok we have a mouse moving.. now we probably need to do something similar for the keyboard..
ok now we have our input devices (lets ignore joysticks), lets figure calculate everything going on in the game world and all the changes that need to be made. Again think about everything in stupid simple operations. you wanted to move forward? ok lets change the position of you, again operation to load that number, and another to load the number you want to increment by, and another to add the two together, oh and now lets move the outcome to a more permanent spot, at least another operations there.. starting to get the picture? oh and lets see if that bullet hit you.. need to compare the edge of the box representing the bullet and you and see if they are inside each other.. ok so lets iterate along the edges and see if any of them are equal (obviously wayyyy dumbing this down). ok for each point i need to load each in and then also compare them.
ok we have moved forward and checked if a bullet hit us but this was all done so far in non graphical space but for this sake lets just say the graphics processor will handle all that we just need to update the changes. oh wait... you wanted to animate how a body moves when you move forward? ok.. lets take all the previous states of the body and parts and then load in the next part of the animation (thankfully an animator told us the paths of the parts beforehand so we dont have to figure it out on the fly!) and add the two together and also send those updates to GPU.
oh and since you want to do at least 30 frames a sec... repeat all those calc/operations at least 30 more times.
TL;DR Here is an article about a supercomptuer (ie >80,000 processors) taking 40 min to calculate roughly 1 sec of 1/40 of the human brain.