It is called the Mac Studio, not the Mac Server.
ECC provides error checking at the expense of a speed hit. For the target audience, speed is more important than the “belt and suspenders” protection of ECC.
Both differences are small, but you can’t fairly criticize Apple for not being fast enough and then criticize them for not deliberately taking a 2% speed hit for something that is, at best, an extreme edge case for the people using it.
Modern manufacturing and improved OSes have made memory failure much less common than it was 20 years ago. ECC still has an edge, but it isn’t by much.
Most work computers do not use ECC RAM as it is more expensive. The work laptops being used are the same Dell/HP/Lenovo that you see at Best Buy. Even the engineering groups crunching numbers are often doing it on normal RAM.
For video editors, graphic designers, musicians, and photographers, ECC is an added expense in dollars and, more importantly, speed that offers no real benefits.
So for the intended audience, it certainly is ‘pro enough.’ While I am sure someone in all the internet can manufacture an example, the statistics show that would be a edge case for a permanent speed hit. If your luck runs that badly, you would be better off spending that money on lightning rods.