It's not just that using the mouse is slow. The mouse is also dumb. By that I mean that you can't tell it to take intelligent actions like jump to/select/delete a particular semantic unit (e.g. a sentence). Using the keyboard allows for more intelligent actions and for composition of actions with (optionally) mnemonic keybindings. The mouse is far less precise and requires some UI for pretty much anything.
Besides, even if I'm thinking more than I'm coding that doesn't mean I'm okay with the coding part being slow.
I forget which program it is, but I've seen at least one document editor where one click places the cursor where you clicked, a second selects the sentence it's within, and a third expands the selection to the whole paragraph.
Combine that and modifier keys, and it's completely possible to have the mouse involved in intelligent actions, just not nearly as widely implemented as one might hope.
Besides, even if I'm thinking more than I'm coding that doesn't mean I'm okay with the coding part being slow.