Disclaimer, I am 1) talking about the US and 2) talking about the current laws not what I think the laws ought to be.
You have to pay taxes on income.
You also have to pay taxes on gifts.
Say someone gifted you something incredibly valuable, like a house. You have to pay taxes on that gift.
Say you don't have the money to pay taxes on the gift, then what?
Say the gift-giver of the house is also willing to pay your taxes.
That would ALSO count as taxable income which you would need to declare and pay taxes on.
Essentially, it's illegal to give things to people whose cash on hand is below a certain threshold.
another part of the reason is that money is power, and building generational wealth dilutes the power of the current elite class. taxes are a very effective way to put a very strong downward pressure on effective, exceptional people originating in the lower classes.