A track guy trying to figure out American football using a soccer reference. Ha.
Why do teams give up 50+ ppg on defense? Easily. It comes down to a few things...A combination of coaching, bad scheming, lack of fundamentals and sometimes you're just simply over-matched on that specific day. Maybe it's one of those things. Maybe it's all of them.
A few things to consider:
One of the key differences is that, like a poster above me said, the QBs have become more agile and "dual threat" over the last probably 25 years.
Another reason is the introduction of the "RPO"...really more of a widespread usage than an introduction...and the spread offense. Take that dual threat QB and add an elite or even good RB and good luck game planning against it. It's hard. It's really hard.
Think about this..on literally every single play, you'll have a 3 or 4 wideout set, a dual threat Qb and that pretty damn good RB. As soon as the ball is snapped, you have 4 WRs in their route and you have no idea if the ball is going to the RB or not. If it does, you have to stop him . If it doesn't, now you as a defensive player have to account for those WRs and the possibility of that QB taking off and running or passing. These are Split second decisions by the defensive players based on what coaching they receive but they still have to remember all that in the heat of the moment..on every play of this type. Like I said, its damn tough.
You also act like teams like these elite teams like Clemson, Ohio State, Alabama, etc are giving up 50+ ppg "every other week". They aren't. Sure you have the Big 12 teams that seemingly have every game like that, but by and large the elite teams aren't giving up 50 ppg more than once or twice a season...if at all.