Well it comes down to what you define as too easy. I mean, you have video games these days that nearly play themselves if you can't handle a difficult section. 5e isn't quite at that point- I have seen with my own eyes that 5e can be very challenging, and that published non-WotC adventures can be as well (I'll plug The Scarlet Citadel by Kobold Press here, it has some very rough challenges).
Heck, even older WotC adventures can be tough, when I ran some of the "updated" adventures in Tales from the Yawning Portal, we lost a few characters because there are some really challenging encounters. But in general, the sweet spot of difficulty varies between player to player, let alone group to group. If WotC feels that lowballing difficulty makes them more money, then that's what they'll do.
There are players who just want to have fun and not put a lot of effort into their gaming- for them, high difficulty and necessary optimization will turn them off of a game in fairly short order.
There are players who have a concept in mind and will want it to be the best it can be.
There are players who will optimize an advantage until it's beaten to death with a rock.
And there are people who make characters in a manner they feel is logical for that character, not simply wanting to take the best options.
One game has to suit all of these kinds of players and the ones in between. How do you make published adventures with that in mind? Sure, you could just present the bare bones of the adventure and let the DM figure it out, but not all DM's want to put in that kind of work- and maybe they simply can't. So what can you do within these constraints without just saying "this is a basic adventure" "this is an expert adventure" "this is an advanced adventure", putting in three times the effort for the same money (as not every group will be interested in all three).
