Gosh, man, it is no matter of being smart but a matter of amount of work. Is it possible to build a whole city all alone? without tools? Try it yourself.
I didn't say it I want to do it without tools. I understand your point, but is not exactly the same. You build a city and your're done with the tools, but a game engine is the base of a game, its foundation; once you finish with it, you're not done with the tools.
Gosh II : "You build a city" ? try to build a city even if you have al vehicles and materials - you probably will do not build one house
A game engine is not just a tool for a game, that is what I'm trying to say and that's how I see it. But I guess you've just wanted to show me how hard and time consuming it is, so I understand.
The problem "with engine" or "without engine" is secondary
because even with engine it is like you are a fellow speaking "It is impossible to build a city alone?, well, i am considering that" and of course you did not do a small booth for a dog.
So reading this (no offence) is like suffer a stupidity buzzing around your (my) head. You got small chance to make something like decent roguelike or quake 2 scale game, if you are genius but the chances are also small