Maybe for you... but I've always been quite fit, never overweight at all, but biking to work on a warm sunny day is a guaranteed way to arrive looking like I just stepped out of a shower with my clothes on. Heck, I can stand still in humid 90 degree heat in the sun, and be drenched after twenty minutes. Plus, most of us don't have shower facilities at work to clean off all the sweat -- do you expect me to lock myself in the work bathroom for 20 minutes while I take a sponge bath from the sink?
And how do you bring a change of clothes if you wear a dress shirt at the office? It doesn't matter how nicely you fold it; it will be horribly wrinkled and creased after sitting in a backpack. Bottom line: it's simply not professional, in a lot of cases. It isn't making excuses, and it really is as inconvenient as people say it is. And I say this as someone who does bike to work, on the few days out of the year that the weather makes it possible to (without sweating, freezing, or being rained on).
(Not saying you should emulate DD otherwise mind!)