These 5 companies are doing, by and large, incredibly well right now. Even if ad revenue is down, the lock-in is sticky at least to some extend beyond the very near term. So now if you look at the Dow Jones (which is not weighted-cap), it's up 2.2% in the past month, compared with the 4.8% of the S&P.
That said, the market is not rational, and I'd be wary of thoughtful statements like "investors think...". With the Fed pumping so much money into corporates (via banks), the cost of capital falls. If the government tells you, you can borrow up to your eyeballs at near zero rates, and we'll probably find a way to bail you out if you're big enough (=too big to fail), that's exactly what you'll do.
Once you've borrowed, you're not just sitting on these piles of cash, you'll invest some in operations, but ultimately what your board of directors wants to see is your stock, so you're going to use the money to buy back your shares. Your company is not materially better in any way, it operates pretty much as before but with lots of debt, but its stock price has gone up-up-up: execs get their bonuses, the board is happy, shareholders are happy.
This can go on for a long time. We've had the longest bull run in history - over a decade long - propped up this way.
[0] https://markets.businessinsider.com/news/stocks/sp500-concen...