Personally, I think it is because search is a important revenue stream for the browsers. It makes little sense to put too much effort in to a feature that will

Personally, I think it is because search is a important revenue stream for the browsers. It makes little sense to put too much effort in to a feature that will limit the usage of the thing that earns them the most. At Opera Software, where I worked for a decade, both speeddials and bookmarks was important streams of income, but it was nothing compared to the income from searches.