Surprisingly large amount to unpack here. First is just marketing techniques, which are not really intended to speak to people's rational minds. More often its about influencing people's feelings by taking advantage of people's looser impressions, in this case the one of purity = good. This isn't strictly rational, it's just a feeling, and the marketing is designed to target this subconscious impression with a quick and easy hook.

Next, from a culinary perspective, a pure ground beef patty does need to be cooked properly, and that's harder than it sounds. You want high heat and a quick sear to get a good crust on it, these Maillard reactions are what you're going for to develop a robust and complex flavor in your beef. Also need a generous salting. Most places fuck some part of this technique up, but when done well it gives the straight beef a certain degree of depth that many find appealing. It's actually counter-productive to this process to add anything with water content, as the extra water will inhibit the beef from quickly reaching the necessary temps along its exterior to get these crusty bits, just by virtue of water's low boiling point.

But, in the event you want complex flavor without having to muck around with very specific cooking technique, you can add complex flavors in other ways. Onion can provide sweetness, Worchestershire can add depth, something like smoked paprika, chipolte or liquid smoke can add a smokey component etc, all of which can add deliciousness. This all has the added benefit of extra nutrition and stretching the more expensive meat into more patties as well. This method is far more certain in its results, since the extra flavor components are already there, making the whole thing much more forgiving when it comes to the actually cooking process. All this really costs you is a few minutes of extra prep work and having to deal with burger purists potentially talking about meat loaf sandwiches.

So, at the end of the day, several viable techniques exist that can produce solid results, and have their own subtle pros and cons. This more elaborate understanding does not fit well from a marketing perspective, however, where the advertiser just wants the quickest and cheapest way to create an impression in your brain, accurate or not.

Thus, ultimately, I fully agree with you.