The transfers from film to video (whether HD or SD) take time and money. The "Cleaned up" part is also not as easy or quick as it sounds. If a film is being transfered without restoration (leaving pots, ticks, and scratches on both the audio and video then yes, all the film needs is a bath and a trip through the telecine converter and tada! a converted film. (this is still a relatively expensive and time consuming process)
But most films going through this process are generally "restored" or electronically cleaned up to remove blemishes, scratches, marks, ticks, lines, and age related wash-out or coloring. Take a look at a film like "The Coconuts" (1929) and compare it to "Little Lord Fontleroy" from the 30s. Both these movies show on TCM. The first has been restored and the latter has not. The difference in the quality of the video and especially audio is amazing. Restoring a film to new condition takes for-ever!!! It is an expensive time-consuming process.
The true restoration and clean-up of a film is done electronically and saved to video (not film). The films restored during the 80s and early 90s are strictly SD restorations. The HD restorations have to be done all over again. Voom showed quite a few restored and films electronically cleaned up.
Again this process is not cheap or quick. I just wish that the HD restoration of "The Days the Earth Stood Still" could have been shown on Voom's Monsters before Dish Pulled the plug!
See ya
Tony