Time Between Verbal Offer and Written Offer?

So I received an offer to internally transition into Management Consulting within my firm. I received my formal offer, and about a week after not hearing back from the recruiter, I followed up with her to check on the status of the offer, to which she responded, "I am still waiting for the final approval for the transfer.  I will follow up as soon as I hear back."

It has now been three weeks since I reached out to her (4 since I received the verbal offer) and decided to reach out to the recruiter again, to which she responded, "I am still waiting for the final approval for the transfer.  I will follow up as soon as I hear back."

I'm having a hard time understanding why it would take any company 4+ weeks to extend a written offer after already extending a verbal offer to a candidate. I'm sitting on pins and needles and the company isn't giving me any information in regards to when the transition will occur, salary, etc.

I'm a worrying to much about this? I feel like there isn't much I can do as the only person who has the answer is the recruiter who won't give me the detailed information. Should I just remain patient? Any advice on what could be happening behind the scenes?