Why doesn't the Paris agreement include emissions from international shipping?

According to the International Maritime Organization (IMO):

Note: The Paris Agreement on climate change was agreed in 2015 by
Parties to the United Nations Framework Convention on Climate Change
(UNFCCC) and entered into force in 2016. The Paris Agreement central
aim is to strengthen the global response to the threat of climate
change by keeping a global temperature rise this century well below 2
degrees Celsius above pre-industrial levels and to pursue efforts to
limit the temperature increase even further to 1.5 degrees Celsius.
The Paris Agreement does not include international shipping, but IMO,
as the regulatory body for the industry, is committed to reducing
greenhouse gas emissions from international shipping.

I've been reading about this and was surprised that countries can pollute as much in international shipping and still meet their Paris agreement obligations. Is there a reason why this wasn't included? It seems odd given that there are so many ships sailing between countries and the numbers keep growing.