There seems to be more interest placed on the ability of a book to sell more than the quality of its words. Most agents shall push for a book only if they are sure it will sell. Most publishing houses shall show an interest in a book once they have verified it has a great chance in their market. This situation presents a discouraging outlook for most budding authors. The fact that you are entering a market that is skewed towards sales more than substance calls for an author to be tough enough to accept it and do their best to get their book published.
When your manuscript does not go far with them, it is best to find ways to publish by yourself. These publishing houses have scouts out there, whose work is to find promising fresh authors for them to sign up. You have better chances of being discovered when you have a book out there. It may not sell as much as you would like, but it shall get you their attention.
There is however a better way you can, that guarantees you better returns on your sweat. You can ignore their offer, which may seem lucrative, but is only a fraction of what your book is worth. You can retain all the rights to your work, which shall fetch you more in the future. This shall call for you to be ready to work hard for it, and to explore your options carefully. Get book selling advice here!
You have the option to join a niche publishers organization, coop publishers, self-publish, or electronic publishing. You may also do an extensive marketing campaign for the book just as you would any other product, with the help of the internet, which makes it easier for you. You shall find certain challenges along the way, as expected. But with the right advice, you shall find coping with the situation to be not as bad as feared. There are websites on which you shall find advice, tips, contacts, a network of authors, and other tools you need to navigate the confusing words of the route to getting established as an author. You can read more about it here. Check out this website at https://www.britannica.com/topic/marketing/Services-marketing for more info about marketing.
You shall, for example, learn how to market the book, from the first page of the manuscript you write to the marketing campaign section at https://www.blog.yourfirst10kreaders.com/how-to-market-and-sell-childrens-books. Most authors tend to write the book first, then think of marketing afterward. This usually means that large chunks of their work have to be rewritten. You shall also get help on how to plan for your book release and sale.