Self-publishers need more than an outlet. There is a huge publishing industry devoted to the publicity, placement and marketing of books. Editors make sure that the books are as good as they can be, agents ensure that the right people get to hear about their clients' publications, get authors onto stages at festivals, and publishers ensure that book launches are as high-profile as possible.
Obviously none of that is going to happen for books that only have the author promoting them. Publishers are discerning about who they take on, and self-published books are very variable in quality and saleability. I know several people who describe themselves as 'published authors', but they are not considered as such in the industry, or by other authors.
Anyone with a bit of cash can publish - having your book selected by a publisher who has chosen it above others and believes from experience that it will stand out is very different. As JamesandJon says, agents will do a lot of work, but again, they are careful to select books and authors who are very likely to sell well.
To be fair, there is often little to choose between a very small 'indie' publisher and self-publishing when it comes to marketing, as small presses have tiny budgets and a lot of the work comes from the authors themselves, such as approaching local book stores to arrange appearances and readings for very small numbers of people. Very different from getting onto The One Show or similar, although that end of things tends to be reserved for celebrities and 'big names'.
Publishers sometimes have submission windows during which they will read work from new authors. There are literary competitions (eg Bridport), and the winners are watched by publishing houses who might approach them. It is worth asking Chat GPT or Google AI who is taking submissions and which competitions are live - you can spell out the genre, length and so on, so the results are as specific as they can be. Good luck.
