SEO is hidden? SEO is the sea that the entire internet has drowned in, unless you know where to go diving through it to find underwater caves that still have some air left.
Can a regular person (or an expert) tell that you see an ad? Usually yes, as it's banner or some visually distinct piece of a web or real estate.
Can one tell that a text is produced by SEO? Usually no (especially, when it's good quality).
Now, the difference is:
- In ads case, a government comes and asks for a tax. Because there is a clear case for a transaction in an economy sense (ad for money).
- In SEO case, no transaction proof is there. Can't come and ask to pay tax, or regulate anything. It could be a happy customer who wrote a piece of text. Or clever SEO author. Or both mixed. Can't tell! In this sense SEO brings less or no tax.
But SEO is quite hidden and hard to define. Ads are there, see a banner. SEO? No obvious 100% criteria for it.