Yes, Dell and HP sell perfectly fine workstations.
No, there's nothing like the current Mac Pro.
A quick look doesn't show a Dell Precision desktop workstation that goes beyond 48GB RAM, for example, while the Mac Pro goes to 1.5 TB.
I also didn't see anything like the MPX Module:
The MPX Module starts with an industry-standard PCI Express connector. Then, for the first time in a graphics card, more PCIe lanes integrate Thunderbolt and additional power provided to increase capability. With up to 500 watts, the MPX Module has power capacity equivalent to that of the entire previous-generation Mac Pro.
Didn't see a way to have up to 8K streams going at the same time or the ability to attach up to 12 4K displays on the Dell site.
It goes on and on. You can approximate a lot of what Apple is offering, but you can't duplicate what they're offering at a lower price.
There's so much Apple-designed silicon in this box, it's literally impossible for Dell, HP or anyone else to compete with off-the-shelf, commodity components.
For the target market Apple is going for, the cost of even a tricked-out Mac Pro is a rounding error.