It normally isn't an issue because of the size of the investments we're talking about, but even if funds did permit lower commitments (through friends and family funds or otherwise), you'd still need to be an accredited investor to partake. This isn't legal advice, yadda yadda.
Being an LP in a PE/VC fund is a much different experience from investing in the market. You don't have a lot of visibility on the value of your investments (maybe quarterly updates but we're talking private companies so valuation isn't as concrete) and you don't really know when you're going to be writing another check or for how much. On top of that, once the funds start distributing you will be taking distributions over many years. Through my line of work I've been able to invest in a few different vehicles and have given up on trying to closely track IRR or anything like that for the class as a whole. If you're comfortable having capital tied up for 10 years or more and believe in your GP's, private fund investments can be a great investment vehicle but they are not for everyone.