As an aside, I find Google's model to be less bad than Facebook's attention capitalism model because it permits a diverse, unoptimised and decentralised web, amp notwithstanding.
Same principle applies to security: each of these trackers has basically full control over the page, and if one of them gets compromised and replaced by malicious code, it would be disastrous. The probability of any individual tracker getting compromised is low, but what you should be worried about is the probability of at least one being compromised, and that probability rises exponentially as you add more.
Let n be the number of trackers, let p be the probability that an individual tracker is compromised, and assume compromises are independent. Then: Prob(at least one comp.) = 1 - (1-p)^n ~~ np. In other words, linear in n. This approximation is fine for small n*p. For large n*p, the probability has to be <= 1 of course, so the real probability is sublinear.
When businesses invest in caring about this (and it can even be just one headcount for the biggest of businesses) there's opportunity to reign it in and maximize the value from a given platform, reduce it the need to add others. That helps with security, governance, and adoption of what's there.
Nevermind more forward looking solutions like server side which gets these injected scripts off your site completely and greatly improves security.