Is there a reason to seek out a low beta fund rather than just allocate more to bonds?

I was entertaining the idea of buying a low beta fund since the market feels overvalued (changed my mind because I have a very long time horizon) but looking for things that would say capture 60% of upside and downside was tricky because different crashes sometimes hit more defensive funds differently.

It occurred to me though you can pretty much capture exactly that by going 60% stocks and 40% cash/short term bonds.

So why look for a low beta/defensive fund and run the risk it doesn’t do what it’s supposed to instead of just allocating more to cash and capturing exactly the amount of upside and downside you want?