"Get 'em while they're young" is as valid for recruiting as it is brand preferences ;)
Those interns will turn into salaried FTEs whose first three year's annual compensation – amortised signing bonus, stock grants, and performance bonus included – will be ~150k. Compared to new graduate FTEs, interns are positively cheap!
The ~6.5k, housing inclusive, perks out the wazoo also all come from highly profitable, competitive companies falling over each other to recruit from a highly constrained pool. There are only so many Stanford, MIT, and CMU graduates a year, and an even smaller number of hackathon winners, open source contributors, inveterate interns, etc. For many, this is the last time they'll ever openly be on the job market.
