I will personally admit this, even as I’m typing this, I’m also part of the crowd as I often neglect my graveyard as a potential resource in my games. The graveyard, which is originally a zone to designate used or discarded cards, can function as essentially an extended hand at times. Additionally, when you use your graveyard, you are allowing for double-playability for many of your cards as you can cast them from hand and then reuse them again.
I think the graveyard is too commonly associated with colour restriction, and that is because the two most common colours that seem to perform any sort of graveyard ability is that of white and black. Black’s most obvious use of the graveyard is through reanimation where they can bring back creatures from there. Whereas, white performs a more nuanced “light reanimation” where they can perform the same thing but often at a higher cost or for smaller creatures.
Every colour has some amount of graveyard interaction to some extent. There are obviously some keywords written in such as that of flashback, unearth, jump-start, and aftermath. Regardless of the activation costs, essentially, these cards give you built-in recursive value to bring back out these cards for at least one more round of fun.
Let’s also consider some keywords that use the graveyard as a resource. This can be seen as in the case of Delve and Escape which transform your dead cards in graveyard into alternative forms of value as a cost reduction for delve and a recast for escape.
Let’s not forget about cards that genuinely care about your graveyard to gain additional abilities. Delirium is a classic that cares about types in graveyard to get additional bonus effects. Threshold cares about the number of cards in your graveyard (seven or more) and grants abilities if you meet the conditions. Considering the low bar requirement of just having cards in your graveyard, these effects reward you for just throwing things away or having things die.
As for the way to get cards into your graveyard, self-mill is an option, but I don’t recommend that you just mill silly-willy; you want there to be a reason to mill. Your cards in your deck are a precious resource, and not all cards carry any kind of synergy within your graveyard. However, in the case of discarding, that’s where things get interesting. What if you were, to say, use your discard as a way of activating an ability. That’s the case for keywords such as Madness and its weaker cousin, Mayhem (I said it, I’m not revoking this). Madness turns throwing cards away into more fun, allowing you to activate the card tossed with the keyword for usually a lowered casting cost, and on top of that, as an added bonus, Madness ignores timing restrictions.
If graveyards are so strong and underutilized, why don’t more players abuse it? Well, the answer to that questions is multi-faceted. Magic is a very complicated game in the fact that there are many aspects, both known and unknown. Furthermore, many graveyard cards do require some kind of set-up to build into them. Yes, even though it is much easier to get cards into graveyard nowadays than say several years ago, it’s not as simple as your basic draw step and getting cards in hand. Outside of some options such as Entomb, it is difficult to selectively place cards into your graveyard. You could go through the route of milling yourself, but results can heavily vary. You don’t want all your cards in the graveyard after all.
Graveyard-hate also exists and for good reason. Given the random and varied nature of Commander, I highly recommend that you bring at least a couple pieces of graveyard hate. Even among graveyard hate, there are many options across the board, each able to be utilized and specified for each of your decks. Do take a look at the many options available.
The bottom line is that the graveyard is an excellent zone that is often underutilized. People view the graveyard as solely a black-based resource. Many are aware of mechanics such as Flashback, but there are so many more than can be used in conjunction with your graveyard. Don’t just let your graveyard “rot”; make use of it as a secondary arsenal that you can blow your opponents up with.