Now, if there are Earth-sized planets out there, shouldn't their
presence have a pretty sizable effect on the orbits of bodies such
as Neptune and Pluto? How can there be such large planets with no
measurable effects until 2017 technology came along?
Short answer: It's not the mass, it's the distance that really matters.
Longer answer: I can't do all the math, but to cover some of the basics. The Discovery of Neptune is the famous example based on unexplained orbital inconsistencies. Uranus' orbit wasn't following Newton's laws and the simplest explanation was that there was another undiscovered planet.
When you say an Earth sized planet, Earth is about 1/17th the mass of Neptune, so an Earth "Mass" planet in Neptune's orbit would have had 1/17th the effect on Uranus' orbit. That might have been too small to notice. I don't want to guarantee that, but it's entirely possible that 1/17th the effect would have gone unnoticed for some time.
Uranus and Neptune's closest pass is about 10 AU from each other. For comparisons sake, Uranus and Pluto's closest pass is about 11 AU from each other. Planet 9 is currently about 20 times the distance from the Sun as Neptune. That puts it (ballpark) about 600 AU from Neptune currently and in this case, closest pass doesn't matter because it's effectively been in about the same part of the sky since Uranus has been observed.
Using the inverse square rule, at some 60 times the distance, that's 1/3,600 times the gravitational effect, and if we give it a mass of about 1/2 the mass of Neptune, that's about 1/7,200th the effect. That's teeny-tiny. In fact, it's not much greater than the gravitational effect that Pluto has on Uranus at closest pass (fun sidebar — Pluto gets closer to Uranus than it gets to Neptune - if this source is right).
And, it's even worse than that because it's not the gravitational pull but the tidal force that should be considered. Any tug that the theoretical Planet 9 gives to Neptune, it gives a similar tug to the entire inner solar system. If an object tugs on everything mostly equally, the observed effect is very small. It's the variation in tug on the outer planets to the inner panets that should be considered, and that's essentially the tidal force, where the 3rd power rule comes into play. That makes any gravitational perturbations a distant 9th planet would have on the inner planets, essentially negligable. Lower than a number of large Kuiper belt objects and that would be basically undetectable even with today's instruments. There's too much gravitational "noise" to detect something that small.
So, basically Pulchritude's answer — the vast distance is the problem. Any orbital perturbation from Planet 9 on the outer planets, Uranus and Neptune for even a large object at that distance is smaller than the effect from Pluto. It might have 5,000 times Pluto's mass, but at 60 times the distance, the tidal force at 60 times the distance is about 200,000 times less. (It's about 20 Neptune distances from the sun, but orbital perturbations are most noticeable at closest pass, and Neptune and Uranus pass within about 10 AU from each other), so that's where the 60 times more distant comes into play.
As Planet 9 moves closer to its perihelion it should become more detectable by orbital wobbles but even at closest pass it's too far for the perturbations to be significant. And it's likely to be discovered (assuming it exists), long before it reaches perihelion.