Because it's a commercial product and Opera's success is partly measured with the market share they hold. Thus it wouldn't be optimal solution to allow their apps to lie that they're Safari on an iPhone when they're not.

Also, usually it's the web pages that need to be fixed instead of building workarounds for browsers. Opera works quite a bit to get web designers realize that browser sniffing is usually very bad idea. Why then support such behaviour with user-agent switching?

Most of the web devs are too lazy and incompetent anyway ( ) to realize that changing user-agent is not an acceptable solution to fix their sites.