Maximum Balance: $2,000,000
An account that offers immediate withdrawals is a risk for the bank. Part of the rationale for savings accounts relies on this being spread between a lot of customers so they don't need to have enough cash on hand to repay everyone's balance.
They don't really want to have multi-million accounts that could cause a cash flow risk.
If you have that much money, they are likely to offer different vehicles for it.
The wealthy don't simply bank like us but with more zeroes at the end. If you're a "$2 million in my checking account is a possibility" type of person, you certainly have an accountant. You have investment advisors managing an investment portfolio sitting in investment accounts (at the very least). You're not putting money in a checking account except when you expect to write checks off of it.
You've heard of (mostly fiction about) rich people who get a ransom demand for $500,000, and they're like "I don't have that kind of money lying around, I'd have to raise it". That's valid. Rich people's money is out working for them, not lying around in accounts.
So a bank would be a little concerned about someone who thought a savings account was a good place to leave $2M. It has no sensible purpose, and it creates liability for the bank - particularly someone hacking the account and transferring away the money. Banks cannot always escape liability for such.
It's also a bad financial practice, due to FDIC insurance. Which only covers $250,000 per bank. As such, if you had $2M to keep in such accounts, you'd spread it across at least 8 banks - or rather, your accountant would.
There are two things to consider.
They can set a maximum amount. The fact that they are still paying the same rate all the way to $2,000,000 is amazing. Some banks will only pay their high rate if your balance is between two amounts. If it is lower they pay way less than 1%, if it is over the higher amount they credit the excess at a very low rate. The range of limits they pick attracts different types of customers.
FDIC insurance only covers the customer up to $250,000, that means that if you are approaching the $2,000,000 limit you are putting $1,750,000 at risk if the bank fails. Very few potential customers will approach the maximum the bank is allowing because they have other options without the risk.