Because buying cheap surplus capacity (spot instances) is likely cheaper than running your own (Amazon has a reason to sell them at any value above the marginal cost of running stuff on them, which is basically power).

They might have a dedicated fleet for some capacity. They might also have a less predictable encoding load (e.g. not be sure how much content they will acquire), or able to make trade-offs (encoding is expensive today? let's do a fast encode and wait until spot instances are cheap with the better one).

They might also be buying for different prices/conditions than what you and I see on the web site.