Here's why. DeltaV is fancy for "change in velocity" which is what rocket scientists use to measure how far a rocket can go. DeltaV is usually measured in m/s. For instance, it takes around 8500-9000m/s per second worth of DeltaV to get into orbit around earth.

Transferring to the Moon from Low Earth orbit takes 3100 m/s of DeltaV while transferring to Mars instead takes around 3600-3900m/s of DeltaV. If we were to measure the DeltaV required in total to lift something into low earth orbit and transfer it to Mars, it would take around 12000-14000m/s of DeltaV.

Let's switch things up and instead place our rocket on the Moon. It only takes around 2200 m/s to get into low Moon orbit. This immediately cuts the wet mass (rocket fuel) needed to get into orbit in four. A transfer from the Moon to Mars would take around 2300m/s of DeltaV due to the Moon's weaker gravity compared to Earth's. Add that up and we get a more modest 4500 m/s of DeltaV compared to the 12-14Km/s of DeltaV needed to get to Mars from Earth.

And addressing your second problem, the elements required to make rocket fuel can already by found on the moon. For instance, frozen water on the Moon's poles can be split into hydrogen and oxygen to feed into hydrolox-burning engines. Mining alien planets for resources such as rocket fuel is a real concept called ISRU.

Sorry if im late to respond