Here's why:
* Suffixes are added to the end of a word to change its meaning or grammatical function.
* "Container" is a noun that means "something that holds something else." The "er" is part of the root word and contributes to its meaning.
If you want to create a suffix from "container," you might add "-er" to create "containerer," which would be a person who contains things. However, this word isn't used in standard English.