A lot of bots love plain and obvious text boxes with big submit buttons right next to them. The easier you make it to fill out a form, the more it is going to get hit by bots. If you've ever worked on the opposite end of a submission form, you've probably seen how quickly it happens and have felt irritated at how they can ruin a good thing (like our Documentation Feedback Form for example).
We didn't design that element of the Search field, but if I were to guess it's probably like that because Guests can search, and it's probably preferable to not have thousands of search queries bogging down the forum server nonstop 24/7 because spam bots are trying to tell you about their new kitchen deals and lovely fake passports. This is my assumption at least. I could be wrong.