In large part, because plate tectonics and plate motion primarily reflect the dynamics of oceanic lithosphere (e.g., Crameri et al., 2019). Within this context, it's good to have a handle on what's driving plate motion, so if anyone needs a refresher, I'll refer you to our existing FAQs on that subject. It's also worth considering that even without any specific mechanism to expect more plate boundaries in oceanic lithosphere vs continental lithosphere (which in fact there are, as we'll cover in a bit), just by virtue of the 70-30 split between oceans and continents, we would kind of guess that there would be more boundaries in the ocean. If we take a tour of the three types of plate boundaries (convergent, divergent, and transform), we'll see that there are in fact a variety of mechanistic reasons for most of these, by definition, to involve oceanic lithosphere. As a result, even ignoring the differences in surface areas between ocean and continents, we would generally expect that a large proportion of plate boundaries themselves would be in or near the ocean.

1. Convergent Boundaries: By far, the largest representation of convergent boundaries are subduction zones and in fact, subduction (and the driving negative buoyancy of subducted slabs) represents the largest force driving plate tectonics writ large. Because subduction is generally limited to oceanic lithosphere (because of the necessary density contrast for subduction to occur and be sustained and its relation to the composition of oceanic lithosphere with respect to continental lithosphere), at least one section of the plates involved (i.e., the subducting portion) is going to be oceanic. Additionally, because subduction requires a strong negative buoyancy of the downgoing lithosphere, this tends to ensure that the average elevation/depth of these regions are sufficient to be underwater (and also are the deepest locations in the oceans). Now of course, there are convergent boundaries on land, i.e., continent-continent collisions like the Indo-Asian collision forming the Himalaya and associated ranges or the Arabia-Eurasia collision forming various mountain ranges between Turkey and Iran. These however reflect former subduction zone boundaries and, geologically, are temporary as there are a variety of forces that actively resist continued convergence as the associated mountain ranges grows and/or the lingering subducted slab of oceanic lithosphere is removed. Individual subduction zones themselves do not persist forever, but they tend to be active for longer, on average, than a given continent-continent convergence system.
2. Divergent Boundaries: Again, the largest representation of divergent boundaries, mid-ocean ridges, by definition involve oceanic lithosphere because they are producing oceanic lithosphere. There are continental rifts, e.g., the East African Rift, but as continental rifts evolve and accommodate more and more extension, the continental lithosphere becomes thin enough that the rift starts become low enough to be flooded by ocean waters and begin forming oceanic lithosphere (transitioning from a continental to oceanic rift), and start to produce a new ocean basin. So again, continental rifts have relatively geologically short lifespans as continental rifts transition into oceanic rifts and begin producing oceanic lithosphere. As an aside, not all continental rifts end up as oceanic rifts, some fail (i.e., they become Aulacogens) and some are never localized enough to transition to an oceanic rift (i.e., "wide rift zones" like the Basin and Range).
3. Transform Boundaries: There are not the same mechanistic drivers for transforms to be preferentially in the ocean as there are for the large classes of convergent (subduction zones) or divergent (mid-ocean ridges) boundaries, but in detail most transform boundaries are also in the ocean because they occur as oceanic transforms between mid-ocean ridge segments (e.g., this diagram). Because transform faults (as opposed to transcurrent strike-slip faults) "transform" motion from either two convergent, two divergent, or mixtures of convergent and divergent, there is sort of a driver for these to be in or near the ocean since so many of the boundaries they link are in the ocean and involve oceanic lithosphere. There are however some other (somewhat related) considerations for why many transform boundaries might be near oceanic lithosphere. One option is that it reflects the tectonic history. For example, the San Andreas fault system, i.e., a transform boundary between the Pacific and North American plate, is in part so close to the coastline because it reflects a history of transition from a subduction to transform boundary. Similarly, many plate boundaries are not perfectly divergent or convergent or transform, but rather have "oblique" motion because the motion of the two plates that meet are not perfectly orthongonal or parallel to the boundary. However, instead of a single fault with a combination of dip-slip and strike-slip motion along a single boundary fault, we often see "strain partitioning", where oblique motion between two plates is split between a dip slip fault and a closely related strike-slip fault. An example of this would be the Great Sumatra Fault, which is part of the plate boundary zone in this region and is effectively tied to the subduction zone that lies just offshore.