I'm gonna hazard a wild guess that it's because it's so time-consuming to finish an instrument that way, it's very slow drying, for starters.
I started a thread on finishes a while back:
http://www.letstalkguild.com/ltg/showthread.php?188931-Finishes/page4&highlight=catalyzed
Which contained this link:
http://www.aitchisoncellos.com/publ...-about-the-cello/understanding-cello-varnish/
Which contains this excerpt:
"However, classical makers resisted innovation in one crucial area: the use of pine resin in varnish. Traditionally, cooked pine resin was melted into a drying oil such as walnut or linseed oil, which formed the basis of most varnishes across Europe during the 16th century. If cooked slowly, the resulting varnish always remains soft and flexible: a beneficial quality for stringed instruments. By the time of Stradivarius, most other woodworkers and makers of other musical instrument families (e.g. keyboard and plucked strings) had adopted more durable finishes made from the tougher resins available at that time such as copal from the Far East, but the great classical makers continued to use their out-dated wood finishing methods and materials with the result that their varnishes have remained delicate and flexible in a way that has enhanced the sound and beauty of these instruments.
If a traditional linseed or walnut oil and pine resin varnish is cooked sufficiently slowly, it will never completely harden, but if you cook oil varnish over too high a temperature, it will harden very rapidly and will then change very little with the passage of time. A varnish which stays hard and glossy for over 100 years is far too hard, and is the antithesis of the best varnishes to have been produced. Varnish which remains soft is in a permanent state of chemical evolution as it reacts with the oxygen in the air and also with other elements which come into contact with it, such as moisture, sweat and skin, as well as changes in temperature. This process never ceases, causing significant changes to the structure of varnish over time, including the extraordinary fact that linseed oil varnish becomes alcohol soluble after about 200 years."
The takeaway from this is that after 200 years you should be careful when sipping single malt around your Collings.