Scrubs are an important medical utility that have been around since about the 1940s and were a necessary step forward in medical practices.
Before scrubs; it wasn't uncommon for surgery procedures to be done whilst the surgeon was wearing a butcher's apron — most surgeons never washed this apron as the quantity of blood stains was somewhat regarded as indicative of the surgeon's experience. This was accompanied by many unsafe and highly unsanitary practices that we would deem to be insane today, but luckily by the 40s, sanitation had become a crucial part of surgery — as it should be.
One Size Fits Most
Originally, scrubs were white to emphasize cleanliness, but so was everything else in the hospital, so after a while of operating or just being in a hospital peoples' eyes would begin to strain. To soften this effect, scrubs were issued in shades of green, and were then referred to as “medical greens.
Green was the color that was picked for mostly optical reasons; it's in the middle of the visible spectrum so it's quite mild, also, blood stains were not as pronounced on those shades of green than almost any other color — in fact the property of scrubs being certain colors that downplay blood stains stays with us today.
On through the 1950's cheap scrubs were very little more than just specially colored cotton garments, and some are still like that today, but with the difference that older scrubs were often ordered on a per-individual basis and were designed to fit well as to look clean and professional. Nowadays there are generally three sizes of scrubs that will fit most anyone, those sizes being: medium, large, and extra-large. (Occasionally with smaller sets as well — but not so often are those used.)
Because scrubs are cheap and effective. They're not designed to be a particularly fashionable garment and do not have zippers anywhere on them, so cheap scrubs are easy to manufacture.
Cheap scrubs are preferred as they're meant to be highly replaceable; you don't want to take any chances with transmitting any diseases to yourself or others — diseases like MRSA or STAPH are particularly easy to transmit and are quite nasty infections.
To avoid this, scrubs aren't kept for very long at all; maybe a few uses, but cheap scrubs are so easy to come by and order these days it's actually an economical precaution to not be shy about safely disposing of used scrubs and ordering more pairs; the cost of having to deal with only one staph infection is much higher than large orders of cheap scrubs.
That all being said, there are some interesting technologies at play in the medical industry as far as the methodologies of maximizing the benefits of using scrubs is concerned among hospitals.
Some hospitals — particularly very well funded ones with lots of money coming in — invest in scrubs that have anti-microbial properties as well as limited permeability along side high rates of absorbency.
These aren't cheap scrubs, and still have to be treated as though they were if they happen to get dirty, but some hospitals feel that it adds to their prestige to have every advantage possible when they have such large budgets.
Though, that being said, some studies show that having scrubs on makes a few doctors and medical professionals feel as though they are clean when they are really quite infected; so some argue that the cheap scrubs that don't make you feel special for wearing them are actually the way to go.
Either way you look at it, scrubs are a vital necessity for hygiene and medical workers the world over, and should remain a part of the medical field for the foreseeable future.