Why haven't we been able to reach distant stars yet ?

I think this is a valid question. It has been 45 years as man last time walked on another celestial body.

Is there something fundamentally missing in our physics ?

Chemical rockets cannot take us to Mars and back efficiently.

Why has DS4G, MPDT and Hall thrusters been overlooked in this ?

Stuhlinger developed the ION-drive 50+ years ago.

One detail in modern physics puzzles me a lot.

I figure it puzzled sir Isaac Newton the same. It is the mass and inertia.
If F=ma ( I don't doubt the equation )....why on earth can we say the stuff weighs 100 kg...while its mass is also 100 kg ? This cannot be right.

Answer to 1st question because its very difficult and will take a lot of energy and money.

Answer to 2nd question weight is derived from its mass in the gravitational field it is in and it made things simple for earth to be 1g so 1 times 100=100 more details here:-

https://en.wikipedia.org/wiki/Mass_versus_weight

This is a bit harmful....as one G= 9,81 m/s^2 so the mass and weight should be equal only on a planet where the gravity is 1 m/s^2.

The ION-drive in space is effective just because the mass of the craft in space is very small.

Also noteworthy and significant is that Apollo could have been much lighter if the second stage structural system was applied on stage 1 and 3. The exhaust velocities and ISP has only developed by 10% since. So only way to cut down the costs is to make them lighter in structure...thus more efficient.