[jacquesm]

Part of the reason is that an organ isn't 'just' an instrument, it is a building which happens to contain an organ. In order to properly model an organ in a particular setting you'd need to model the building as well and that is getting into a wholly different level of complexity.

Pianos are more or less designed to sound 'the same' (with some obvious differences between major brands and some pianos that have been constructed very explicitly to sound different, such as the Klavins instruments).

https://klavins-pianos.com/products/

So within reason you can model a particular piano just by specifying make and model and look up a bunch of parameters in a table.

With an organ in a 'virtual' church you could specify the exact disposition (the number and kind of sets of various pipes and other complications) of the organ, and you could supply some basic parameters of the space (size, area, placements of the ranks and so on). But I suspect that if you did that you'd not get even close to what the real instrument in its actual setting sounds like. Organs are super temperamental, air pressure, humidity, temperature, intonation, various stuff that is broken (almost no mechanical organ is ever in perfect shape), mechanism sounds (which can appear very different even within the same make and model) and so on.

This is a stupendously hard problem and as a happy user of Pianoteq I'm impressed with Modartt even attempting this and I really hope they will succeed in being able to switch seamlessly between different venues and to place an organ of a different manufacturer into an existing church (just to give one example of what that would make possible). But I'm not going to hold my breath for it and sample based organs (including the free GrandOrgue) still hold a significant edge compared to Modartt's organ simulator. But for piano it is now so good I almost prefer it to the real thing for practice. Not that I'm such a discerning user, more that I've grown up with pianos and they're always (unless very recently tuned) a bit out of tune and that can be quite annoying. The software based one is always perfectly tuned and sounds as good or even better than the best sampled instruments (personal favorite from the sample side of the aisle: Yamaha P-515 Bosendorfer sample set is extremely good).