those idioms and slang still follow grammar rules though. they are not irregular.

also the irregularities in english developed over centuries by contact with other languages in an environment that lacked the normative influence of globalization and modern communication tools.

at the time english was also not the defacto language for international communication that it is today. this status is responsible for efforts such as a wikipedia in basic english that makes information easier to approach by non-native speakers.

as esperanto was expressly created with the goal of being an international language, any attempts to introduce irregularities will meet a lot more resistance and therefore development of an irregular grammar is unlikely to happen.