But why? Why have anacondas - and sharks? - been immune to evolving? Why hasn’t a significant predator evolved - or invaded? - to feed on them? Why hasn’t 12 million years made the species fragile?
That being said if the question is "why have we not seen significant morphological changes" - there are a few ways to think about this.
First is that we would be blind to many types of morphological evolution. For example, if an isolated sub population of snakes or sharks started shrinking due to isolated environmental pressures, we would be unlikely to see this, but also if we fixate on the "largest anaconda", then we would filter out all "smaller snakes".
Second, the way we talk about "not evolving", especially for sharks is probably misleading. When we say sharks haven't changed, we mean to say that the shark body plan hasn't significantly changed. And this makes sense - they have a very efficient body plan for being a hunter in the see. We have "proof" of the suitable-ness since dolphins and other whales have converged onto a very similar body plan. Conversely, there are plenty of extinct sharks with body features that seem totally bizarre (https://www.fieldmuseum.org/blog/four-fossil-sharks-are-cool...).
Finally, especially in the context of "the largest" - the largest animals that can exist in a given environment is.... environmentally constrained, especially for land animals. The largest anaconda is likely near the largest sizes that the local environment to support, and so something larger appear is unlikely, without drastic environmental changes.
In any case, 12M years a long time for a species to survive, let alone survive “as is”. It makes me think of the creature in Alien and how it evolved into deadly perfection. But these creatures aren’t fictional.
p.s. Aren’t octopuses another species of little to no change? But they’re weird anyway so it’s not a surprise?
An additional complication is that some cephalopods have a relatively unique ability to change gene expression in response to environmental factors [0]. As a result, even if one were to see physiological change or change in ecological niche, it might not be as a result of speciation.
Hopefuly soon techniques of analyzing ancient DNA [1] will be more broadly used to understand the stories of long surviving species.
0. https://www.nsf.gov/news/masters-acclimation-octopuses-adjus...