Lab employee here. I can't speak for all labs but here's how it works at mine (note that we do both photographic and press printing as well as a few other methods).

It all depends on the type of equipment that is going to be printing your product. Traditionally photographic prints are done on a minilab or equivalent piece of equipment which prints in RGB. When you order a flyer or a business card it is going to be done using a press which prints in CYMK. There's many other ways of printing beyond photographic and press but those will be the most common.

While it is true that one can convert between the two color spaces there's certain colors that simply can't be converted. Take a look at the following image for an idea of the different gamuts that exist for various color spaces:

If you were to send an image in RGB to be printed on a CYMK device where there's colors that fall out of the gamut of what CYMK can achieve, a substitution will occur that attempts to get close to your intended color. This can cause your once vibrant and beautiful RGB image to come out looking dark and dull on a CYMK printer for example.

For this reason we suggest customers send us their images in RGB format when a photographic product will be produced and CYMK when ordering a press printed product. In reality most of our press printed work is also sent to us in RGB and our customers feel the output is satisfactory. Not all customers feel this way and those that want truly accurate color do send their press printed images in CYMK color space.