Strads and Guanarius violins are worth so much for a combination of reasons: 1) They are very collectable and much sort after, and 2) they are rare. There is a finite number of them in the world of which at any moment in time a very limited number are available to buy. So simply it is ther laws of supply and demand in action again. The demand for them far outstrips the supply, so the price is high. With there being a finite number (no more can EVER be made) and every year there are losses to this pool of Strads - accidents, wear and tear, theft etc - and consequently the value is likely to increase. This means that they have brecome collectable not only as great musical instruments but as an investment. When this happened their value took anotherr leap.
I collect violins myself (as a musician) and although I don't have any Strads or Guanarius in my collection (yet!!!) I very much enjoy what I have, and over the years I have expanded and most importantly improved the quality of what I have. This I continue to do. A selection of them can be seen on my website - http://www.virtuosiviolins.com