Xjyuk

Recently, I heard of three tourists from Germany drowning in a lake in Alaska. Their boat overturned and they ended up in the water. All three of them were wearing life jackets. This is what I don't understand, how can people still drown if they are wearing a life jacket?

Life jackets do not make one drownproof, just increase your odds significantly.

Our data also show that over 80% of drowning victims were NOT wearing life jackets when
found. We know from other data that most of those victims could have been saved had
they been wearing a life jacket before the mishap occurred.

But, you ask, what happened to the other 20%, the ones who were wearing life jackets,
but drowned anyway?

...

Those are the primary reasons boaters wearing life jackets sometimes drown. Either the
boater is unable to free him- or herself from some type of entrapment, is unconscious or
otherwise unable to keep his/her face out of the water, or eventually drowns from
numerous mouth immersions over a prolonged period of time.

The Other 20% --
When Wearing a Life Jacket
Is Not Enough
By CDR Kim Pickens, U. S. Coast Guard Reserve
Operation BoatSmart Project Officer

Of course you are way better off wearing a life jacket but it won't prevent you from drowning in all cases. The other killer if you fall off a boat in the ocean is hypothermia.

In the specific case you mention, both the NY Post and Deutsche Welle say that the cause of death is still being investigated, and both of them point out the extremely low water temperature (the victims were kayaking in a glacier lake). DW mentions that they were not wearing any protection against cold water. At the time of writing, hypothermia seems more likely than drowning. The NY Post article mentions a water temperature of 9°C, corresponding to an expected survival time of 1-3 hours. Considering that the victims were in their sixties, the lower end of this range seems more probable in this case.

PFDs come in various flavours:

The best ones have sufficient floatation around the neck, and enough more insulation on the front compared to the back, than an unconscious victim is naturally rotated onto his back with is face out of the water.

The 2nd tier will float you upright, but if you are unconscious, your face may fall into the water.

Third tier are flotation belts which may or may not keep you upright but reduce vulnerability to undertows.

The first time you have to work at drowning in. It happens. An unconscious victim in choppy water can drown from repeated splashes in the face.

Most boaters are using 2nd tier ones. They are way more comfortable to wear casually. Regulators figured that a great lifejacket that isn't worn is not a win.

In Alaska, I'm betting that hypothermia is part of the picture too. In cold water it quickly becomes impossible to take effective action to save yourself, and soon after mental ability deteriorates.