Summer jobs don't exist because of LMIA. Teens do apply for jobs but employers just never look at the applications or place any calls/hold interviews because they know they want LMIA workers right off the bat because LMIA workers can be easily exploited and abused because they don't know how our labor laws work, some of those laws don't protect them, they can be entirely beholden to the employer. LMIA is modern day serfdom. Its practically slavery. Everybody is criticizing international students instead they should criticize the whole LMIA fiasco.
That plays a role, though summer jobs, particularly for High School aged teens were on the decline long before the surge of Temporary Foreign Workers and Foreign Students.
A portion of that owes to higher HS graduation rates and grater emphasis on good enough grades to pursue post-secondary. Meaning, fewer teens are pursuing summer employment, and for a period of time, those jobs
were very difficult to fill.
You have then have different responses to that phenomenon, automation, (think ordering kiosks at fast food places or self-check-out ), various offices and the like simply phasing out summer interns (paid and unpaid), then layer on to that...
The surge in TFWs, plus moves away from lawns towards 'natural' landscapes, so fewer landscape jobs; plus, a surge in population that means there is greater competition for the jobs on offer; all with a currently faltering economy.
***
Note for others, LMIA is 'Labour Market Impact Assessment' and is what is historically required of employers to demonstrated a need to import TFW labour because the jobs aren't fillable domestically.
The Trudeau government both made those easier to obtain, and also relaxed various protections resulting in downward pressure on wages.