Many publishers receive 100 unsolicited manuscripts per day. There are 120,000 new books published every year, about 10,000 per month. It takes about 18 months to get a book to market after negotiations are completed. A typical first print run is 5000 books. Most books lose money.
Big publishers take control away from the author.
If a big publisher considers your work, it is unlikely you will have control over it or make enough money to pay for your time. An Expensive Way to Feed Your Ego Vanity publishers produce about 6000 titles per year. The publisher doesn't care what kind of book it is. The publisher cares about being paid by the author. The publisher has no investment and little incentive to make the book a success.