Question

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

Answer 1

Answer : When resistors are connected in parallel, the supply of current is equal to the sum of the currents passing through each of the resistors.

To be specific, the currents in the branches of a parallel circuit add up to the supply current. When resistors are connected in parallel, they have the same potential difference across them.

When resistors are connected in series the total resistance is greater than the individual resistances, hence the currents flow is less.

When resistors are connected in parallel, more current flows from the source than would flow for any of them individual resistors, so the total resistance is lower.

Answer 2

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.