It may worth mentioning that you don't really increase your security with increasing the number words to more than 12.
A 12 word seed phrase provides the same security as a bitcoin private key.
My high level thinking - as I don't quite grok the detail is this
The clue in hosseinimr93's reply is in the second sentence.
The private keys that it generates have a security of 128-bit due to ECDLP. (
explained in this topic by nullius)
The logic is: since the derived private keys have 128-bit of security, there's no major advantage of using a seed with higher security.