Enterprise buyers are used to the effort to get a quote. And they want to feel special. Combine that with how different they are (is enterprise 100 users, or 10,000?), and you can see why they aren't sufficiently similar to treat in one plan. The high dollar value also means you can afford to have a high touch sales process.
Normal customers often shop on price. Enterprise customers often don't. You're selling to the normals, and sending a signal to enterprise folks that you can handle a customer of their size.
It's not about pulling out a credit card and just paying for the service, but about multiple people in the customers company having to approve of the purchase (including the team manager, their manager, the CIO, the purchasing department, legal, etc.)
One of the key parts of an enterprise salespersons job is to help the individual at the company who wants to purchase the product to navigate their own internal purchasing process.
Enterprise sales can often involve more work in-term of onboarding and ensuring your capacity scales to handle them correctly. Enterprise customer will also often want to negotiate pricing and have custom legal terms drawn up (NDAs, etc.).
Because of all these factors it doesn't make sense to have an upfront price because the sales price will vary from customer to customer.