Yes, naive attempts at manipulation will be detected these days on big platforms. 5+ years ago such naive attempts were successful, though. A few years ago I made a proof of concept to show how easy it is to make new Reddit accounts and automate them, and had registered hundreds of accounts. Those logins still work even though Reddit has cracked down on naive automation attempts.
Today, that's why many firms buy real users' accounts. They'll hire people to manually login to the accounts and post. There's also the perpetual cat and mouse game between bot creators and platform owners, and the platform owners who benefit from the appearance of increased growth and engagement that's actually just bot activity.
How they verify those outcomes, however, gets interesting. See, for example, College Humour going under due to inflated engagement metrics fed to them by Facebook video.
More than that, you misunderstand how advertisers prioritise their money. They pay for outcomes. If they notice that over the past couple of months they’ve been receiving mostly bot traffic, they stop advertising. Not everyone all at once, but enough that revenue begins to decline. An ad based business that cares about the long term will do it’s best to weed out the inauthentic engagement.