You're assuming an awful lot about router manufacturers: mainly, that they are good at best-security practices. Case in point: my router (a linkysys) doesn't have a name on the password field, because it turns the form into an ajax submission that passes the password over BASIC authentication. It doesn't do this over HTTPS (a terrible choice), and it also attempts to turn off autofill on the password field (another terrible security choice). (Now I remember why I bought a router compatible with open source firmware: time to go figure that one out...). So truly: don't assume that router manufacturers have any idea what they are doing when it comes to security. edit I had previously said the password on my router was encrypted client-side before sending. I realized that the security was so bad there was no way they were doing that. So I double checked. It wasn't encrypted. They were just base64 encoding the username/password combination to make it easier to transport.
As @ISMSDEV mentioned, your suggestion is just security through obscurity. As a general rule of thumb that is not the best method to secure anything. Security through obscurity can deter simpler automated attacks, so it isn't crazy to try it, so long as your application is otherwise secure. You certainly wouldn't want to worry about it as your only security method (which obviously isn't what you are suggesting).
However, all things in software involve a cost-benefit analysis. Introducing ad-hoc security methods is as likely to introduce more bugs (and more points for vulnerabilities) than anything else. More security is always good, but you need to make sure that any given security measure has real benefit. Adding in more code to maintain with marginal benefit is probably going to hurt security in the long run.
Presumably, most router manufacturers don't consider such a security feature to be worth the effort. Then again, they don't consider even basic security to be worth the effort. shrug