I'd have to agree with everything stated above. If cubits were easier to earn, then everyone would be MVP+ without having to spend a penny (possibly an exaggeration on my behalf, but I hope you understand my point). It'd be completely unfair for people who spent IRL money to get a rank/item, when they could have waited and got it for free by grinding cubits.
As
@KatrinaLightning said, voting daily will give you 0.10 cubits, and within a week, 0.70. If you have synced your discord, there are also daily rewards you can claim, and if you claim all 7 every week, the 7th reward is 0.50 cubits. Depending what server you play on, you are also able to trade cubits for mana/other items with players who are willing to trade. For example, Olympus; 0.01 cubits = 2,000 mana or a Pig Spawner. I don't know the conversion rates on the other servers, but that's just an example of a common trade on one of the servers.
ManaCube is the first server that I've seen at least to offer an in-game currency which you can use to buy in-game ranks/items, and I feel people sometimes take that for granted. Of course, I understand not everyone is able to spend $50 on a rank for a Minecraft server, but if you really want to purchase a rank, you should be willing to put in the work in-game to earn it. I understand it can be time consuming, but it's still at least an alternative to paying IRL money.