Advertising is a very typical arms race. If you're the only one advertising, a small cheap ad is very effective. If your competitor is advertising you have to spend just as much just to tread water. If you spend more, it forces your competitors to spend more too.
It's not surprising that the modern Silicon Valley economy is built on advertising.
To give an example of that - my friend runs a one-man internet shop, selling niche items. His monthly profit is around (an equivalent of) $1-2k, while his Google invoice is $7k$-10k. So, essentially most of the fruits of his labor go to Google. And, according to him, if he tried spending less on advertising, he'd just vanish from the face of the Internet and customers would buy at his competitors.
It's not surprising that the modern Silicon Valley economy is built on advertising.