Because IFR time has no relevance as a pre entry qualification to be able to learn how to teach flight by sole reference to instruments!
I think you have completely missed the point here. The IRI allows someone to teach for the issue of an IR. An IR is not just about flight by sole reference to instruments, that is just one (important!) part of it.
An IR (of some description) is required to fly IFR; unlike the old rules someone without an IR may not fly IFR in any met conditions. But on the other hand, they have the privilege to fly in controlled airspace anywhere in the world. The whole argument about the IMC rating or IR(R) centres around this being excessive to allow a PPL to get home in UK weather.
To have adequate experience to teach for this, adequate experience of all aspects of IFR flight are required. Someone who has spent hundreds of hours tracking to and from their local VOR in Class G airspace while under the hood may be excellent at hand flying with sole reference to instruments but is hopelessly inadequate to teach IFR procedures.
Anyone who thinks that that modern airliners are not being flown on instruments just because they are not in cloud is hopelessly wide of the mark. They may not be being hand flown, but the crew are most definitely determining the flight parameters by sole reference to instruments. Indeed, one of my hardest tasks as a trainer on modern jets is getting the crew to look out the window when they need to (e.g. class E or below airspace!).
Don't get me wrong, I also have tremendous problems with people who struggle to hand fly - but that is as much the fault of companies who encourage use of automatics to the max, since these guys presumably were able to hand fly well enough to pass their initial IR at one point but have not been able to practice and consolidate.