Why did my iPhone not come with a plug?

Hello ~ It is intended to reduce the number of charging blocks going out into the environment.

iPhone 13 Pro and 13 Pro Max - Technical Specifications - Apple

In the Box

• iPhone with iOS 15
• USB‑C to Lightning Cable
• Documentation

You can use one you already have or purchase in a variety of places. Take a look here:

Power & Cables - iPhone Accessories - Apple

~Katana-San~

The assumption is made that most customers already have a house filled with power adapters, vehicle USB ports, car chargers, computers, USB built into power outlets/strips/etc. But it's going to be a simple thing to buy a new one. I believe most are still using USB-A power adapters with MFi cables. I have a home filled with them that I bought cheap.

About Apple USB power adapters - Apple Support

As part of our efforts to reach our environmental goals, iPhone models no longer include a power adapter in the box. iPhone continues to work with existing power adapters and USB-A to Lightning cables, such as those included in prior models. iPhone models now include in the box a newer USB‑C to Lightning cable that supports fast charging and is compatible with USB‑C power adapters and computer ports that you might already have. You can use either a USB-A to Lightning cable or the newer USB-C to Lightning cable with your iPhone.

Apple no longer ships phones with cords to cut down on packaging. Most of us have many such cords available. You can also buy a cord if you don't have one that will work.