PPC ads
You end up paying more for PPC ads because you're paying every time a user clicks your ad (hence the name pay per click).
While ads appear at the top of the search results page, there is limited real estate. Since there's limited ad space with PPC ads, you're entered into a bidding system that rewards the highest bidders and drives up the cost of the ads. With the rise in popularity of PPC ads, most PPC ad platforms like Google Ads and Facebook Ads have become more expensive. Another thing to keep in mind is that roughly 30% of people have ad blockers today, and an even larger percentage are skeptical of ads, believing that they trust Google over an ad placement.
SEO
SEO as a strategy is much more affordable and yields a higher ROI since traffic from organic search using SEO is free, so you're not paying for clicks in a bidding system. People trust Google and organic search results much more than they trust ads, with most people avoiding clicking on ads altogether, or installing an ad blocker. While there are more advantages to SEO than there are to PPC ads, it's important to keep in mind that SEO takes time, patience and work since visibility for desired keywords isn't quick or easy, but the results are worth it and come with a higher ROI than PPC ads.