Intentionally so because trying to sharpen for output is not a visual activity...it's trial and error at best and believe me when I said a lot of trees died to make Lightroom's output sharpening work well. Unless you can state exactly what you think is missing from LR's output sharpening, I don't see the above statement useful. It's limited to those sharpening routines we think are useful. Facts would be better than opinion...
I'm not sure, but maybe you interpreted my remark in the sense that LR sharpening is bad, or something. All I said is that the control over the sharpening is limited. You ask for facts because you value them higher than my opinon, I'll give you facts.
How about sharpening for printers like a Noritsu or Fuji frontier? Laser printing on photochemical paper is different than printing on an inkjet printer, it can benefit from a different type of sharpening, and they do not all have the same native PPI resolution.. The same goes for certain types of inkjet media, Canvas knows many different structures, and it's quite different from Glossy which is different from non-glossy paper. Perhaps we want to print on non-glossy paper, yet convert it to glossy with a Diasec treatment, or coat it with resin. Or perhaps we need to print it on self-adhesive material or ... I do know there is some limited control/choice in LR, but then I didn't say it was bad.
Another fact is that interpolation, upsampling, usually doesn't add detail (and hopefully few visible artifacts). What few people seem to understand is that the upsampling itself can be mathematically characterized by a Point Spread Function (PSF). That fact can be exploited by using deconvolution sharpening after upsampling. It can even be taken as far as skipping the capture sharpening, thus avoiding any risk of artifacts and enlarging those, and combining the capture sharpening with the output sharpening with a (somewhat timeconsuming) restoration of detail, called deconvolution at the native print resolution. This would help in producing huge magnifications e.g. for tradeshow booths. A good friend of mine shoots & produces huge (6x3 metres or larger) background images each year, often from a single 21 MP 1Ds Mark III frame, and they will be viewed from close up when people come to the stand after stopping in their tracks as they come walking down the aisle when they see the images.
I'm glad you now agree that upsampling to 720/600 PPI has it's merits, after an initial reluctance against my long time suggestions that it can produce better output (depending on subject), because it offers more control (e.g. better interpolation than the printer driver and sharpening at the native reolution). So let's now keep an open mind that while LR has improved (good enough for many), there may still be a need for more user control (perhaps outside of LR, yes there is a life outside LR ...).