why do we have 100+ MPH on our vehicles if we arnt allowed to go that fast?

Because it's the most efficient and economical way to build a car. The engine performs most efficiently in the middle of the range so if you want it to have the best fuel consumption at, say, 50 mph, it needs to be built to do over 100 mph. Then at a legal speed, it will be purring along just nicely and using the least fuel per mile. Like you doing a gentle jog, instead of running as fast as you can and getting tired quicker. A less powerful engine would end up costing you more in fuel to go at the same reasonable speed, it'll be loud, and probably go wrong more often because you're pushing it to its limits.

Then what about hills? Go up a hill and you'll want that extra power, or your car will slow to a crawl.

Why are you unable to watch your speed and keep it legal? Nobody's forcing you to put your foot down.

You're allowed to go 100 in Canada.

Having extra speed/acceleration available can be very important in certain emergency situations

Go to Germany.

I know what you're on about and its a very good question. In japan, all or most cars are limited to (restricted) 180kmh (112mph). The thing is most of the speedometers in the cars max out at 180kmh yet if the restrictor was taken out they can go alot faster. But like here in the uk the limit is 70mph yet the max number on the speedo is 140mph. Sadly i don't know why but thats just an example

Does your foot have a mind of its own and presses down as hard as it can? Anyway, there are good reasons for it. The 100+ on vehicles is measured on a level surface. If a car was purposely made to go a max of 80 mph (which is legal in some places), when you climbed a mountain pass, by the top, you'd be going a whole 30 mph.

Speed limits vary by location. Cars are made for international sale.
In Germany there is no speed limit on much of the Autobahn.
You can track day your car.
At lower speeds the car is not working hard giving better fuel economy.

Did 120 mph in my Pontiac Bonneville convertible. Let up on the gas pedal when the car started to feel light. Did this in a straight back road in Florida where there were no speed limit signs posted.

" ... why the government allows us ...''

That's your problem.

It is up to you, not the car, not the government to operate your vehicle in a safe manner. Any ticket I ever got, it was not the fault of the car, or the cop, it was my foot, and I own my own foot,
Vehicles require scant little HP to cruise on a flat surface, but require magnitudes of order more to accelerate to any given speed, pass, climb hills, or tow loads. That all translates into an ability to go quite fast as well.
Many highways in the US West, have speed limits of 80 mph, and some had no limit until recently. The traffic volume is minuscule, and the distances are fantastic between towns.

Don't know and DON'T GIVE A RAT'S BACKSIDE!