This is arguably a question about search engine algorithms, or perhaps about culture, rather than language. But since this question is still not closed, I'll take a stab.
You see freestanding bathtubs suggested for the same reason a search on hardwood will return images of flooring instead of trees, and a search on brushed will show metallic textures on appliances or fixtures and not hairstyles or paintings: it is "spring cleaning" and home improvement season in North America (and you do not appear to be using a regional version of Google). Home renovation in general (real and dreamt-of) has been greatly popularized in recent years thanks to a confluence of trends from home improvement television shows to the availability of demo videos online to constraints on the housing market and economy at large that encourage renovation instead of relocation.
Note that what people type into search engines is not directly correlated to what they speak or write. There is no particular reason to believe freestanding in a conversation will lead any participant to think of bathtub — except that freestanding is not a word most people use commonly, and the place where it would appear the most is probably in relation to home improvement, and there are only a few freestanding things in relation to your home, and among these a bathtub will be the subject of more conversations than, say, a lanai or carport.
First, in Canada and the United States (I am not sure about elsewhere in the world), the majority of household bathtubs installed since the first quarter of the 20th century have been built-in bathtubs, designed to be mounted on a frame or platform and set against a wall.
As noted, there is a broad interest in home renovations, and kitchens and bathrooms offer the opportunity for dramatic changes, the better to show off to your fashionable friends and to cite as a reason for inflating the value of the home. Because the built-in style is so commonplace, one way to stand out is to replace it with a freestanding bathtub, either in a modern style as in the screenshot you provide or with a more old-fashioned aesthetic.
In contrast, most furniture and appliances in the house are freestanding. There is no need to refer to a freestanding ottoman, or sideboard, or secretary; that is assumed to be the normal state of affairs. Thus, for things which are ordinarily standalone — notably shelving — a home seller may underscore that they have been installed as built-ins as a differentiator. By the same token, things which are ordinarily built-in — such as bathtubs — are differentiated as standalone.
Outside of the home, there are few references to freestanding except for trivia about the tallest freestanding structures; within the home, freestanding is going to refer to a handful of items, of which bathtubs are probably the most universal.
Tangential to this discussion is the concept of the retronym, a term coined because a formerly unambiguous term has been made ambiguous due to new developments in the world such as technology. For example, the term live music was invented to distinguish musical performances from recorded music. Before the phonograph, of course, there was no such thing as recorded music, so any music played was by definition live.
While the standalone bathtub is the older form factor, the built-in type became the predominant form, and what an American considered to be a standard bathtub. While the term built-in (like drop-in and various others) has long been used in construction, it has entered popular usage because of the new necessity to distinguish the built-in type from the standalone type. At some point, fashion being what it is, that may revert, and search engine algorithms being what they are, Google and its ilk will suggest different related searches to try.
freestanding *
(I limited the results to 1950 and later). You see structure, columns, building, wall, but no "bathtub". People search a lot for "freestanding bathtubs" is all the Google images result tells you.