As with so many things, the devil is in the details. Here's a simple keyword example:
accounting services and accounting service are different, with substantial gaps in search volume and competitiveness. Do you know which one is worth more? Do you know how much more?
There's an order of magnitude difference between the traffic on nj accounting firms and new jersey account firms. Are you sure you know which is better?
Even if you cultivate a list of hundreds keywords that are relevant, valuable and that you can be competitive in, do you know how to decide which words to try to hit via SEO and which to do via SEM? Do you have a tool that can help you make that decision quickly?
Do you know who your competitors are, what keywords they're winning on, and how strong they are on each, so you can plan an effective attack, leaving them only with the crap you don't want?
Sure, it's pretty easy to say use valid html, use alt tags, use meaningful URLs, have a header, use relevant images, make videos, get links from relevant content with good anchor text, and all the other crap that goes into SEO 101. But when you get into market analysis, competitive analysis, management of link-building campaigns, etc, it gets very involved. I'd expect that it would take smart person a year of working full-time under an expert to be good at it.... and I'd expect it to take them much longer if they're doing a part-time self-guided tour.