Because as you described you have to try it to find out how much it will save you, by which point you could probably just roll it out already.<p>Say you have 10 servers now and you expect business to grow by 10x in next year, so you put 6 months of dev. time into implementing caching. Then you get 10x the traffic in a month and you get V.C. investment as a result. You then have a problem, people are getting a bad experience and the development won't be done for another 5 months. Your CEO buys a rack full of servers, but you can't do anything with them because you don't have a scalable system. Alternatively you could find out after 6 months that you can't make that 10x performance jump after all.<p>Twitter had this problem a while back, essentially their DB server got overloaded but it was already a 8 CPU, 64GB machine and despite having lots of money they couldn't quickly solve the problem, because you couldn't really get a bigger system.<p>Facebook knows how to scale, they want to cut costs where they can to become (more) profitable.