The Uneven March of Progress Bars

Why are the progress percentage bars in programs so useless? This one in TrendMicro’s HouseCall has been telling me it’s 99% done for more than an hour:

This is by no means unusual. Software, whether it’s loading, installing, booting, scanning or whatever, doesn’t seem too hot on the old timing front. The jump from 0–99 is always impressively fast; then the last percentage takes forever. Sometimes the percentages and time remaining are disarmingly precise, but bear absolutely no relation to reality. You’ve got to feel sorry for people who plan their day around a progress bar.

OK, my question is this: Is this just bad software, is it just me, or is it really hard to compute how long software is going to take? If it’s the last, why bother? Why not just put a window up saying something like, “This is going to take a while. Go grab a coffee, go jogging, engage in some form of intercourse, and then check back in. Whatever you do, don’t watch this progress bar because it will lie, lie and lie again”?

Or is this some scam? Does the computer know the whole ‘99% done’ thing is rubbish, but was there a dastardly plan devised by the software company’s marketing department to convey an aura of efficiency, speed and precision by making the software seem really, really fast, at least in its initial phase? This, the thinking might go, gives off a very favourable first impression which is only mildly dented by the growing haze of soporific depression that overcomes you as the progress bar sticks at 99% for the rest of the workday?