It s about the sense of responsibility, attitude and maturity level... Its common for 21 yrs old to give fk care knowingly they themself as an intern. Literally a 21 yr old and fresh junior grad have same intellect level to carry out job, difference is the factors above. Heck even a 26 27 yr old hop to a new job know zero, he will assume more responsibility and be more serious than an intern in learning.

Think back of it I don't blame seniors for not putting interns at 100% effort. They are bound to leave, the scope of responsibilities has a limit as well, and it's more of a risk to waste effort guiding important tasks for interns rather than simple ones. Even myself can't trust a lazy technician, what's not an intern? That said I've intern two places before, one was at accenture. Both places there's limitations in the extent of the tasks, and I admit I had my fair share watching movies secretly also. But that didn't mean I am a lazy ass, I performed well but just that my maturity level was to play at the age of 21. Just admit it only 10% of that age don't take things seriously. Like my peers who were the same, came out successful in career also.