Because you can’t just boolean search the entirety of the web the same way you do a local database. You’re not getting all results every time you search, that’d be insanely inefficient, so doing full filters like with boolean database search won’t work.
That said, based on my experience with Google, negations work just fine, as do double quotes. Last time I checked NEAR even worked pretty well. AND is implied, OR used to work but is probably derived from the rest of the query these days.
People hate it when their query doesn’t return anything. So, whenever you search for something and get very little results, search engines will relax their boundaries to find something that may answer your query.
Search engines in the early 00s had them because they required very specific phrase matches and the experience was horrible. You either got millions or results that didn’t relate to what you were searching for or you got none and had to start over.
At some point, search engines started interpreting what you mean instead of what you type. For most people, searching for “rain” and getting results about “precipitation” is exactly what they want. Using the 90s/00s search term syntax, you’d need to type “~rain” to also get synonyms, which is obviously a terrible user experience that serves only the most pedantic people.
DDG doesn’t have their own search index, they’re mostly using Bing and maybe a few fallback providers. They’re entirely reliant on other search engines that they purchase results from, they barely do any actual searching themselves. Things like image search even come from one single source (Bing).