Meat of a healthy animal is, by definition, sterile. That is why it is so important to get the intestinal tract out undamaged. So healthy meat is not contaminated from the start.
Of course, in the meat industry, especially of certain nasty jurisdictions, there is little hygiene, and the animals are often sick or fed antibiotics, or both (due to breeding resistant bacteria). Often, it could not even be sold without prior disinfection.
And of course, if you take it out, it will instantly get contaminated on the outside in any non-fully-sterile environment. (Including the skin and intestine!) The question is just how much?.
And that depends on the temperature, humidity and amount of light, as well as porosity and damage (rips along the cutting surface), but meat has blood vessels, lymphatic vessels, pores (esp. chicken), and other means to get into it. Also, the bacteria of certain environments can handle lower temperatures. (This is why fish needs be colder than fridge temperatures: Their natural environment is often already that cold.)
So the bacteria will, over time, get in there. And the more they can eat in there, the more gaps there will be for more bacteria.
Now of course, if you wash it hard enough, you can theoretically wash those bacteria out there again too. But as you can imagine, over time that has to get closer and closer to just destroying the meat really quickly. Like … hours. (Imagine your meat being marinated in soapy water, then washed out, and repeat. If it then still tastes of anything, it will taste of soap. :)
(To be precise, some things aren’t soluble in water nor fat, and therefore can’t be cleaned with soapy water. But this is such an exception for anything living, that I think it it is negligible for this situation.)
In any case, if the butcher is actually serious about food hygiene, and the farmer is actually serious about keeping their animals healthy, almost any meat can be eaten raw for a certain time after. (Bats being a critical exception, since their immune system is so extreme that they just aren’t affected by most pathogens, and carry them happily.)
This is common practice in may countries. Japan has sushi (raw fish). France as steak tartare (raw minced beef). Germany has Mettbrötchen (raw minced pork). In Germany, meat cannot even be sold if a veterinarian doesn’t give their approval before and after butchering. So even minced meat is sold for raw consumption on the same day, and consumed (on bread) a lot, as a breakfast or snack.
So my real answer would be, that washing it is unnecessary, and based on assumptions stemming from already bad (and here in Germany illegal) practice by those who sold that meat.
This was a major conflict during “free trade” negotiations between the EU and USA, for example.