Why so expensive?

It is possible to build cheaper, but if you then can get a better second hand boat for the money, it will probably not sell..?

Part of the problem is all the added extras that have been added to boats in the last 20 years. Electronics everywhere, drop down/pop up TVs, $2000 sound systems, pods, joysticks, thrusters etc. This cost big bucks.

then of course all the emission control stuff. Sure it s great to have outboards that do look like mosquito sprayers in steroid but that’s $$$$$.

and the list goes on

You've omitted a couple of zeros

If your just going to live on it, get a house barge. If you want self propelled, you ad lots of $zeros.
When you relize a large boat may have two engines, the endless list of equipment to marinize and operate them makes up for many $zeros of any boat. Then light plant(s), A C, galley equipment. Then sanitation.
Not even started on comforts yet.
Think of any safe, self propelled craft as a space ship. If you want to cruise in aqua space, then you need all that Capt Kirk had and it cost $ with lots of zeros.
You just want to get on the water a few times of the year? Then rent a boat.

What most of us have done, purchase a not so new boat that is still serviceable. Not with all that glitter and new style, but while on the inside looking out, the view of aqua space is the same.

It's not just the price of the boat, it's the money it takes to maintain them. And if you do it yourself, it takes a lot of precious time. The joy of owning a yacht is certainly there but it doesn't come without significant costs.