why do film makers rent expensive cameras?

"I also know that many great Hollywood films have been shot with DSLR. Such as the Nikon d800 and the canon 5D MK1."

[Citations needed]

There may have been some independent productions shot on DSLRs, but few major releases.

"What I am dying [t]o know is why would a Hollywood film maker allocate ridiculous money to rent a 50,000 + dollar camera when they could simply shoot the.movie with a DSLR capable of 4k video?"

That's a naive statement.

Big budget productions call for equipment that gives *complete control* to the production team (director, cinematographers, editors, etc.). They do not want settings pre-determined by engineers who have never been on a movie set. No compression, no automatic adjustments, no wonky jumps in zoom (people are specifically hired just to operate the zoom ring on these cameras), no risk of the equipment itself compromising the shoot. The resulting video is of much higher quality than the format for which it is destined, yet also incomplete and will be edited for months afterward.

A DSLR might have a little dimple mic built into the camera body, or a 1/4" mono mic jack. Real audio production is done with dedicated audio equipment to ensure that absolutely no whirring or vibrations from the camera itself are heard.

As fhotoace mentions, there are plenty of business and legal considerations that the producers have to deal with.

Don't confuse any of this with the very real possibility of productions being done on a lighter budget using equipment from Black Magic, Red, or other such brands. But your "ridiculous" cameras operate at tiers well above these.

They use their production companies and film looks tons better than digital. If you use digital it would look like a camcorder.