Why are leaders of certain countries underestimating current situation?

One of two reasons i imagine.

The first being that they don't want to stop the flow of money from business into the economy.
As it is many companies are on economic 'life-support' as the rest of the world effectively shuts down. Making the the transfer of goods or services impossible.

The second is to prevent an inevitable panic from their citizens that they would have no way to stop.
If they introduce martial law they would then be responsible for the welfare (feeding and medical) of an entire nation !

Because politicians aren't the best scientists (to put it mildly) and scientific advisers might feel intimidated about being upfront with their leaders due to the severity of what's expected to happen?

Many of them feel the need to be “in control”

Control-freaks are typically weak minded

There’s been a worldwide phenomenon in the last 30 years of electing well connected morons.

Herein lies the recipe for denial

I am no fan of politicians. Any of them.

Currently, though, it's a balancing act. It would not be helpful for a leader to lean forward to the mike and scream "The sky is fallin'!"

The logistics of this event are mind-blowing.

what are leaders supposed to do exactly ? - seal off the boarders - that's one

Because they care more about than their ideologies and agenda than about citizens

Because it might make them look bad.

they want to balance their people's approval with their safety.
Crack down too hard and people push back, too little and people suffer needlessly... and push back.
Politics is a popularity contest.

It's an old and common practice, not a new phenomenon, and is usually a rather forlorn attempt to bolster their own citizens' morale and the nation's appearances abroad.

I think they do, it isn't prudent to add to the fear. Calm is needed.