I'm one of the ones with an extravagant present (a cruise) upthread.
HOWEVER we live a very frugal life mostly based around the kids, and we hadn't been away just us for more than a couple of nights since the eldest was born (she's now nearly 8).
So it was really a holiday for both of us, plus a nod to the fact that we'd spent really no time or
money on ourselves really since the kids were born. Our parents live too far away to be regular babysitters, so we don't even get regular nights out.
My 40th felt like a bit of a recognition that our decade had mostly been spent in service of others and it was a good time to mark it. My 30th was far less extravagant!
I think that may be why a lot of people have big 40ths - perhaps financially a bit more stable plus recognition that celebrations may have been few and far between with child rearing.