The tricky thing there is that the page can nonetheless update after we make the bitmap without our necessarily being aware of it. Or even the act of bitmap-making itself can cause an update. Or the bitmap can be a slightly different size from the page, or positioned a bit differently (e.g. assorted wackiness with iPhone X edge margins / notch, etc. And Apple have introduced a major breaking change in web view scrolling in I believe every major iOS update since we launched Web Reader, so it's something we'd not only have to untangle the first time but likely spend a lot of time untangling yet again on every system update.
Honestly I think the best solution to all of this is to do less looking-up of stuff on web pages and use Reading Mode more; all of our paid e-books (a collection that will soon be expanding greatly) work in Reading Mode exclusively, and EPUB will shortly have that option too (and it's already how we do EPUBs on Android). We have no intention of getting rid of the option for direct tap-lookups on web pages anytime soon, but in the longform reading use cases where minor aesthetic nits like this would likely be the most bothersome I think we're much better off investing our time in expanding Reading Mode support rather than trying to hack UIWebView into presenting stuff in a more aesthetically pleasing way.