Their “official” reason (as shown in the included screenshot from Apple.com) for not including a charger block (or earbuds) is environmental. It allows them to reduce the size of the box the phone comes in, thereby allowing more phones to be packed and shipped in one container. This reduces shipping costs. It also results in less material being used to manufacture these smaller boxes. Saving resources. They even tell you on their website to keep your existing charger and earbuds or buy new ones. Also, it’s not just the iPhone 12. ALL previous versions of iPhones will switch to this new reduced packaging without the charger block or earbuds.

Unofficially, in addition to the above cost saving measures, they are saving more money by not including these items for free. This allows them to spend a bit more on the phone production costs but keep it at around the same price as previous models. You may have noticed the other thing that was pushed at the new phone reveal….MagSafe accessories. By eliminating the charger plug (cord is still included) they are hoping it drives more people to buy the MagSafe wireless chargers and other MagSafe accessories.

Given that phone updates and restores have pretty much moved to over the air procedures, and with the improved MagSafe wireless charging, don’t be surprised if the charging port goes the way of the headphone jack soon and is removed completely in an upcoming model.

EDIT: Some people don’t seem to understand what “official” means. This is the reason that they are presenting to the public. It’s meant to show that they are doing something good (in this case reducing carbon footprint through reduced packaging and shipping) to take away from the negative impact on the consumer (need to purchase a previously included item). If you read the entire answer, and my subsequent replies, I clearly get into how this is more than just an “environmental” decision. There are obviously other business/money making reasons that went into this choice.