I would expect that nearly all private investors simply don't have enough money to buy a reasonable quantity of all the shares needed to run a viable tracker.
If you had £50,000 and decided to track the FTSE 100 that would be £500 worth of shares per company. Buying each company would incur dealing fees of around £10 per company (100 * £10 = £1,000) which is 2% of the investment and another 0.5% for stamp duty gives a total of £1,250 in costs.
If you were using a different investment strategy you would probably spend this £50,000 over say 10 companies which would incur dealing fees of around £10 per company (10 * £10 = £100) which is 0.2% of the investment, stamp duty is another 0.5 giving £350 in total.
The difference doesn't seem that great but being blasé about the odd thousand here and the odd thousand there is likely to lead to losses not gains.
Given all the work involved and fees incurred I am left thinking that the only reason to run your own tracker is that you want to track something that there is no product for.
This could be something recognised such as the FTSE AIM 100, the top 100 companies on the Alternative Investment Market or an index that you have created yourself.