There's a difference between an authentic photograph,made in camera with a lens that imparts a particular type of "lens drawing", and a Photoshop fake. One is authentic, the other is faked. If one has to ask the question "why", it's clear one has not studied the issues involved for very long.
The Lensbaby's images will ALWAYS be distorted, less-than-perfect,and there are a few subtle effects that are exceedingly,exceedingly difficult to Photoshop in, such as the presence of severe color fringing on say, the original Lensbaby. Photoshop techniques create "similar" effects, but they do not include some of the optical problems,like the aforementioned color fringing, that gives the original Lensbaby images their distinctive look.
It boils down to wanting to do authentic "photography", where the goal is to make photographs by using a camera and a lens and light to capture a particular type of image; the Lensbaby can be used with film or digital cameras. The Photoshop way is for those who wish to re-arrange pixels,later at the computer, and who are largely afraid of committing a less-than-perfectly sharp image to the capture medium. The old-school crowd understands photography as an effort to translate a vision directly to the capture medium (film or digital), while the new-age crowd is afraid to commit to a single, set type of rendering, and wishes to "keep its options open" so that if one thing doesn't work, an second,third,fourth,fifth, or even twentieth Photoshop rendering of the original image can be tried until something decent is finally, "created".
This is a pretty simplified summary of the differences between old-school photography and the new process of "digital photography". One is focused upon striving for a finalized creation of an image with a very limited parameter adjustment potential, while the Photoshop crowd makes basically NO commitment to the original image, and relies upon the computer to finalize the image. To the very visually sophisticated, it's usually quite easy to spot Photoshop fakery that attempts to imitate the Lensbaby look; some in-camera lens or filter techniques, like using in-camera diffusion, cause a diffusion of highlights into the shadows that can expand the dynamic range and which cannot be exactly replicated in post-processing; the original Lensbaby's high degree of chromatic aberration for example, is not replicable in post-processing. If one wants a cheap effect, Photoshop allows a lot of tries for adjustment, but if a person wants to "stamp" an impression on an image indelibly, then one selects a lens that has certain characteristics--either "good" or "bad" characteristics. The "drawing" or "rendering" of different lenses is one of the more nuanced areas of photographic appreciation, which most beginners and most intermediate photographers are totally unaware of. The idea that the Lensbaby's effects can be "easily simulated" in post processing is true as long as your appreciation of lens drawing or lens rendering is at a low to intermediate level.