Why can't we just keep the currency at the same value? And why does it only happen to money?
Because money does not exist. We only have Bitcoin that has the potential of being money some day but for now all we have is currency. (currency is medium of exchange and money is a currency that is also store of value).
Why is it like this? Because economy as a science is wrong. What they teach kids in schools, universities, etc. is wrong. Which means the economy that rules the world is built on top of flawed sciences hence it works like this, where the so called "experts" believe that inflation is a good thing.
a. population increase
if more currency didnt get printed, generation growth would happen but each person would have less cash each
You are confusing supply and demand with inflation.
Printing money (increasing fiat supply) doesn't automatically mean inflation. Where that printed money goes can lead to inflation. They also don't print it based on population, they print money (or should print money) based on demand which could be simply referred to as the "production capacity".
But in reality they print money to cover their budget deficits which doesn't go into production so it causes inflation. For example US doesn't print $1 trillion every 100 days because US population is suddenly growing, or the US economy is growing. Neither one of these are happening. What is happening is a massive budget deficit that they have to cover by printing that much money.