Why do most EFI scemes use look-up tables? Why not multiplex engine sensors and compute on the fly? Is there some sort of timing issue?

A few reasons:

1) Ease of adjustment, either manual or automatic, it's much more straight forward to adjust data in a tabular form than to try to "adjust" a mathematical model
2) Resources, either human, or CPU, model based systems require the model to be developed (well beyond most hobbyists), and then that model must be executed in near real time to gain the responsiveness required - something most embedded CPUs to this day aren't fantastic at or simply cannot do
3) Pragmatism vs idealism, models feel nice to the nerd within, however tables, even really low resolution low quality ones, provide ample real world performance both in power output, economy, and drivability - why do anything else?

Hope that helps :-)