My understanding is that most of his wealth is held in MSFT stock which has increased in value by over 4x since 2016.
So if he had given away 75% of his wealth in 2016, he'd still have the same net worth today.
I think Gates is probably to the point where it's functionally impossible for him to give away all of his money. The value of his personal brand and network is such that startups will probably give away decent chunks of their business just to be a part of that network.
Also, completely liquidating and giving away his assets would result in a large payout once.. and then nothing ever again. You need money to make money, so holding onto some to appreciate in value (investments, interest, etc) results in more money being donated over a longer amount of time.