I would argue that with rates that low Berkshire should have borrowed more money

Berkshire already had surplus accumulated capital plus ongoing cash generation. That meant more cash than could intelligently deployed so additional capital was not a problem that needed an urgent solution. If he were doing a large transaction I am sure he would have used additional borrowing to improve the returns based on the prevailing cost of borrowing but that wasn't the case last year.

He borrowed in Japan because he had clearly identified yen based investments lined up with a dividend yield on that portfolio alone significantly more than the interest costs. However, the size of the borwwing was based on the size of commitment to the sogo sosha basket.

Buffett is not a fan of leverage to earn a marginal bit of return anyway so it is not a surprise this was not a priority for him in the last couple of years.