- Time restrictions: They just didn't have enough time to invent a better casing. And out of practicality - and maybe because current users are already using and daved the space for this form factor - they keep it.
I would suggest it's more calculated than that. They've done this every time they make a significant change to the internals. The first Intel iMacs had the exact same design as the PPC ones, the first M1 MacBooks and Mini looked the same as the Intel ones.
I think they do it to make a point. When they switched to Intel, there was a lot of anxiety about the switch. People were uncertain what it meant for them. Will stuff break ? Is it still a 'real' Mac ? Then they deliver a machine that looks and works exactly the same as the existing iMac. The vast majority of users cannot tell the difference other than that the new machine is much faster. It's the exact same UX, it's the exact same Mac, just with better performance.
Same when they released the first M1 Macs. Same anxieties from users. Same way of dealing with it. The new MacBooks and mini looked the same, just faster, quieter and with better battery life. It's still the same Mac you know and love.
Then, once people have decided they love the new hardware, they can change the way it looks.
They need to do the same with the Mac Pro's as it's a different market than the existing line of Mac's. Mac Studio had a little overlap, but the real high-end users of Mac Pros are still going to be very wary of the radical new architecture and what it means for them. So they deliver a machine that will just be a drop-in replacement, business as usual. Wait a year or so, then drop the user-visible changes.