Welcome to this site and the area of investing. The question you are asking, believe it or not, is behaviorally based not mathematically. Would this fund still be a good deal if you got a 10% average return? Yea, very much so in the current interest rate environment.
Now the obvious point is that many people, such as myself, cannot purchase this fund as I am not authorized to trade in the UK. No biggie, there are similar funds here in the US that offer similar rates of return.
So why don't people put all their money into this fund? Well some years stock funds have bad years and significant value can be lost. If you were forced to sell at that point, then you would lock in your losses. However, what happens, is that over time people recover and are rewarded with gains that average pretty high. So for money that you might need in the short term, it should be in something stable, boring, and that does not earn much.
So why don't people put all extra money in this fund. Imagine two people one puts all their discretionary income into stock funds, the other spends all his money on cars and clothes. Who "looks" richer? The second person. The first person probably is not much better as they probably have a miserable life, but over time will have a whole lot of money. It takes time to get there, and sometimes it is tough to stick with the program when your fund is down 25% over the last three months.
This fund, and ones like it, is a facet of wealth building. One must save some, give some, and spend some to properly build wealth. Of the savings some needs to be in a simple and boring savings account, and some needs to go into accounts much like the one in question. However, it is easy to focus on too much of one facet so we must strive to have a balanced life.