Why my laptop chargers never last?

What you are doing is moving around 
in a way that causes it to get spun up 
(e.g., always turning one way to put it 
down, then turning the same way when 
you pick it up). This causes the cord to 
get kinked up. Never pull hard on the 
cord in order to unkink it... unplug it at 
both ends (from the socket and from 
the laptop), pick it up and untangle/
unwind it, then plug it back in. Do this 
every few days (e.g., before going to 
bed or when you first grab it in the 
morning), OR learn to not always be 
spinning the same direction. Lastly, 
don't slam or kick the power converter 
brick around -- it might be a good idea 
to use an extension cord, or get a longer 
version of the charger cord, such that the 
brick always remains in one place on the 
floor (or wherever it normally lays). 

Probably, because you are putting undue strain on them.  Cords rolled up improperly, allowing the charging port to take weight it shouldn't, allowing the plug in collection to be damaged in transit to and from.  I've had chargers for my laptops that have lasted 10 years!  AND they are still working.  When I'm done, I roll up the cable carefully, wind up the electrical wire carefully, always keeping care not to let any extra twists get in it.  I never use the charger while the laptop is on my lap, or on a bed, etc.  I use the charger to charge or power the laptop only when it is on a flat surface where nothing with interfere or come into contact with the charging port/plug.  

It might be you. All the laptops I bought, brand new and secondhand, are still in their first and original chargers. The one I bought brand new is already 8 years old.