It isn't, it is expensive and time-consuming to collect this data, and the edge is huge (I know a fund manager who has been spending several million a year for the best part of a decade collecting historical financial statement data). Also, a lot of the large databases are known to be inaccurate (for example, CRSP). I think the best long-term data is Credit Suisse DSM and Global Financial Data (I can't attest to the accuracy of the latter but have heard it is good).

The website says it is using Schiller's data...so the model is missing almost everything. Stock markets have existed in multiple countries for centuries, using one country makes no sense. It also looks like the bond data is totally wrong (prices rather than total returns).

I was thinking about making a monte carlo simulation for my own use. Obviously I'm not going to be spending millions for the data. What's the best freely or cheaply (i.e. not "call us for pricing") available historical data for stocks and bonds? Or is the free data just so bad it's not worth doing at all? And what are the typical problems with the free data?