Why is this?  Why are more American Citizens are NOT being trained to do these jobs?

In a lot of cases Americans don't want to do these jobs.  

There's a myth among anti-immigration folks that "Americans won't do these jobs" is itself a myth.  The idea is that if these jobs paid good wages then Americans would do them.  But in many cases that's not true.  Many of the "essential" jobs that immigrants, both legal and illegal, do are in the food harvesting and processing industries.  Many of these jobs pay relatively decent wages, or at least can depending on productivity.  But Americans still don't want to do them.  Historically, there have always been jobs that Americans mostly don't want to do, no matter what the wage.  Americans, for instance, have generally been reluctant to work as domestic servants.  So for much of the 19th and 20th centuries domestic servants were usually either foreign immigrants (who didn't necessarily share these prejudices) or African-Americans (who lacked other economic opportunities).  The same thing is true today.  Very few Americans want to go out and pick cabbages or cauliflower, even if the pay is decent.  These are jobs that they see as beneath them.  And most Americans are not willing to do the backbreaking labor that these jobs often require.  In addition, most of the agriculture industry only needs large numbers of workers for a few weeks or maybe months out of the year.  Immigrant laborers usually string together a living by traveling around the country to different areas where picking season is occurring, giving them year round employment.  But few Americans want to live that kind of nomadic lifestyle and few have the ability to work for just a few weeks during harvest time.  Food processing jobs are more stable employment, but it's often physically demanding and gross work.  People may need to stand for hours in a factory disassembling pig or chicken or cow carcasses.  Most Americans find that kind of work disgusting and unappealing.  In almost every food processing industry the workforce is dominated by immigrants, legal or illegal. 

First, thanks for a VERY good question.

Secondly, you do not post this Yahoo headline for the rest of us to read the article.

A lot of immigrants, documented and undocumented come to America and readily take the jobs that a lot of regular citizens are unwilling or afraid to do.  these jobs need to be filled and they pay well...but you risk your life.

Even among LEGAL immigrants - why did Trump prevent hundreds of LEGAL doctors, just when we need them most? 

https://www.propublica.org/article/hospitals-are-s... 

Illegal immigrants take jobs few in America are willing to do. You wanta go be a line butcher in a packing plant? My great-great grandfather came to this country as an apprentice butcher - that's the way Legal immigrants came back then, also for jobs nobody wanted. 

Because you're a fat, lazy and unmotivated slob

Americans probably don't even apply for most jobs as farmhands, food service staff, or hospital sanitation teams. If you need a job, you know where to look...

Mostly these  jobs are care jobs that require people to work with the elderly and demented.  The jobs do not pay well but require people who are kind and patient and clean.

most people dont want to work slave wages for 10 hour days 

It does not take training to be essential service when that service is farm labor, it takes stamina and the willingness to work long hours for minimum wage.