Demon Beasts generally do not prioritize dignity over life as humans do, especially those with a bit of intelligence; they treasure life exceedingly. Besides, these two Demon Beasts have clearly not cultivated for long, far less than Mengmeng's cultivation time, and their wisdom is even further incomparable to Mengmeng. They reached today's Rank due to the special cultivation environment of God Burying Valley, which is countless times faster than outside.
In other words, several hundred years of cultivation can match thousands of years of other Demon Beasts.
However, rapid physical cultivation speed does not equate to the evolution of intelligence.
Otherwise, Mo Liancheng would not have subdued them so quickly.
Consider that Mengmeng can fight Mo Liancheng to a draw.
Mo Liancheng used medicine to seal their cultivation and then threw them into the Space Stone.
After that, Mo Liancheng bypassed the Hero Tomb's monument and arrived at the back mountain.
