To use legitimate interest as a reason to process data you need you be able to argue that you do actually have a good reason to do so and that the user would expect you to process it.
For example, I think that websites have a legitimate interest in anonymously tracking your browser behaviour to analyse performance data and errors so that they can improve their app.
The loophole is that advertisers use it to process way too much data (when they are pretty much the reason for the bloody law in the first place) and that nothing is done about it.