The other answers here aren't addressing what I believe to be a core misunderstanding within your question.
You said:
So when someone made an NFT of the first Twitter posting,
It wasn't 'someone' that made an NFT of the first Twitter posting. The Twitter founder made an NFT of the very first tweet ever, which was also made by the founder of Twitter. The entire concept around NFT's are tied to who made it, and brushing it off as 'someone' is misunderstanding why some people are attracted to buying them.
If you made an NFT of the first twitter posting, no one would care. It would be like you signing your name on another artists painting.
Now, why do people spend money on NFT's? A mix of reasons. Some is Tulipmania where people are speculating that they will be worth more in the future. Some are interested in the concept of NFT's as a way to support artists and prove patronage.