It's probably largely historical.
Most apps for most phones were distributed primarily via carriers for quite a while, and they've monetized virtually everything since day one. Even most ring-tones cost money (often twice over -- pay once for the ring-tone proper, and again for downloading it).
Contrariwise, pretty much since Microsoft decided to start giving away copies of IE, all browsers on PCs have been free, and (to be honest) most have been working hard to maintain market share even though they are free. Extensions have been seen by many as a way of "selling" the browser itself, and were largely given away to help gain market share for the developer's preferred browser(s).
That leads to a lot of inertia as well. Given the large (huge?) number of really good extensions that are already free, I suspect the number of users who'd even consider paying for extensions is pretty small. The field is already pretty crowded, so you'd need to do something quite spectacular to justify any higher price.
I believe the Google app store (for one example) already supports a pay model at least in theory; given the number and quality of free extensions, however, it's hard to imagine an extension gaining many customers at a higher price.