Why is there no plug to charge my new iPhone SE. I have no means of charging it with the cable supplied

See the following from Buy iPhone SE - Apple

What’s in the Box

• iPhone SE
• USB-C to Lightning Cable

Our environmental goals.

As part of our efforts to reach carbon neutrality by 2030, iPhone SE does not include a power adapter or EarPods. Included in the box is a USB‑C to Lightning cable that supports fast charging and is compatible with USB‑C power adapters and computer ports. 

We encourage you to re‑use your current USB‑A to Lightning cables, power adapters, and headphones, which are compatible with these iPhone models. But if you need any new Apple power adapters or headphones, they are available for purchase.

If you don't have any USB power cables and adapters and don't have any USB-C power adapters, one option is Buy 20W USB-C Power Adapter - Apple

Many folks already have the required adapter from previous models, so Apple stopped supplying them with each iPhone as a method of limiting electrical waste.

Purchase your adapter here (if you're in the U.S.):

Buy 20W USB-C Power Adapter - Apple