People can. If you download and install Zorin OS Core, you can then assemble the essence of Zorin OS Ultimate on your own. Every part of it is Open Source. There may be one layout that the average user may not be able to set up, but an experienced Linux user should have no trouble.
Zorin OS Ultimate is expertly assembled and as an option, removes the hassle of not just installation, but conflict-free meshing of apps. But it exists for the purpose of Supporting the ZorinGroup and development.
The Zorin Group does Zorin OS full time. And the Zorin Group should be able to earn a living and be paid for those hours of hard work.
I am not one who puts the Zorins on a pedestal. I am sure they put their pants on one leg at a time and I have disagreed with Artyom Zorin on a variety of issues, some quite publicly.
But it takes even the most casual of glances at Zorin OS 16 to see the hard work, dedication and care put into its development. By just Two People.
That Zorin OS 16 includes Panel Settings, Software Sources, etc. shows that User Voices have not only been heard- but diligently answered.
Choosing Zorin Ultimate is a way users can support the developers, keep the project alive and give payment where payment is due. It's not about the Ultimate OS, it is about Zorin OS as a whole. It is honor based, primarily because generally, people are Honorably Donating to the Zorin Group by choosing Zorin OS Ultimate. It is not necessary to restrict it, since it is what it always was- a "Thank You" to the developers and a great Operating System for the user.
Most users that visit this forum are clear as to why they chose Ultimate. It was not a need. It was not necessary. It was because they believe in Zorin OS. It is a way of supporting FOSS to Keep FOSS Around.
For a Business, this hits even closer to home. For a Business, a city counsel, a school; They can receive discounts on Zorin OS Ultimate. This is fascinating, given the purpose Zorin OS Ultimate serves... and it goes to show that Zorin OS Ultimate is not "bought". It is "Chosen."