The barrier to entry for advertising was pretty small when Google entered it. They worked hard on pricing models, ads relevance, ads quality, and making it unobtrusive to users (in the beginning). That's what allowed them to gain traction. The barrier to entry in advertising is much higher now, but if you have a standout product (like TikTok) then it's possible to gain enough eyeballs to get you through the initial period with advertising.
