Why do two resistors in parallel together contain less resistance than the same two resistors in series?

The easiest way to understand that is to think about a 1-lane highway that's
1 mile long.  If millions of cars are at one end of it, it can only carry a certain
number of them, and the rest have to wait their turn.

Now build a new highway, exactly the same as the old one ... 1 lane wide and
1 mile long.  Put the new one next to the old one.  Connect them together at one
end, and connect them together at the other end.  Now each car that arrives at
one end has a choice ... take the old road or the new road.    In a manner of
speaking, the drivers now encounter less "resistance" in their attempts to get
from here to there ... exactly half as much, in fact ... and twice as many cars
can get through now in the same amount of time.

That's the way I like to explain the idea of parallel resistors.  It always helped me
understand them, and I hope it helps you too.