The issue with hiring many juniors is, when there is another dev boom, all the juniors that were hired, now mids or seniors, are going to jump ship to whoever is going to pay them the most. So, the company can either grow talent and then pay them market rate or hire at market rates from other companies that grew them. Hiring juniors, while good for the industry in the long run, doesn't really benefit an individual company. It is game theory and it is still why senior developers make a lot of money despite there being an oversupply of juniors.