Because they can constrain the design of the language, and make it more complex to reason about the language. Look at the language necessary to explain their p

Because they can constrain the design of the language, and make it more complex to reason about the language. Look at the language necessary to explain their pitfalls in the C standard, or the extra copy needed in operator++(int) to support post semantics in C++. They also make the tools that consume that language slightly more complex. It doesn't sound like much, but in aggregate it's the difference between writing a parser for Java vs. a parser for C++.