Keywords has always been unreliable as the default search. The default should be changed to the title.
I agree that the default should be changed to a title search, which is probably the more expected option. (I suspect that more than a few people don't know the difference, and their searches suffer accordingly.)
A keyword search would be stronger if people were consistent about contributing keywords well.
Sometimes people post a thread without using any keywords at all (and as few topics as possible). Sometimes, someone has taken the effort to add a number of "amusing" keywords without adding any more obviously useful ones.
Even better approaches have their limitations. Sometimes, someone adds only keywords that aren't in the title, possibly under a misconception that words in the title automatically appear in a keyword search.
I know of another forum that has a rule about adding a certain number of keywords (which can be phrases with spaces) to every new thread. Although I'd hesitate to implement a similar rule here, I appreciate some of the intent behind it.