It also gives the sellers time to find a new place/get packed and moved.
The lender gets their ducks in a row because they're going to package and sell the loan, and there are lots of compliance issues to jump thru to get it sold (properly) after what happened during the crash. Lenders are very careful now, verifying down payment sources, income, credit, etc.
I also did cars for a while and I can tell you most (all?) in-house car financing is provisional, and they do the hard work AFTER you drive off the lot. They like it this way because once you've parked that shiny car in your driveway and shown your friends, you'll work hard to keep it should something come up with the financing. If it doesn't work out they can (at worst) tow the car back to the lot. Not quite that easy with a house ;)
On the other hand, people often pay extra for a lender that has a history of closing on-time.
We made an offer on $2.2M house last year in July, and needed a $1.3M mortgage and $500k HELOC (so $400k down.)
We had a fantastic agent at Wells Fargo who promised he could pull it off in 15 days, at which point we'd leave on a 3 week vacation out of country.
We signed 13 days later. The guy would call us at 10pm asking for more documentation when more information was needed.
However, we had to send a copy of our plane tickets to prove to approvers further down the pipeline that there was a justifiable need for urgency.