Because such treaties aren't quite as binding as they may seem at first glance, and helpfully include clauses intended to allow a little wiggle room.

First an analogy:

 - Civil law contracts and agreements drawn up unilaterally often throw 
   in a pile of "gimme" clauses that ask or imply requirements exist for many more things
   than the signers must *actually* legally provide.  A slight
   exaggeration:
   
   > *Rumpelstilzchen Inc.* also requires the firstborn child of signer within 10 days of birth via FedEx courier for processing to be sold
   as unguents... this is a binding contract ... bla bla bla ... These
   Terms are **void where prohibited by law**
   
   Of course the firstborn child requirement is prohibited by law and therefore not legally binding, and
   therefore is **void**, but every now and then some signer who didn't
   read far enough innocently provides *Rumpelstilzchen Inc.* with free
   feedstock anyway, so it's sometimes profitable for the drafters of
   contracts to leave all sorts of junk clauses in, followed by an
   overarching **void** clause.

End of analogy.

For treaties, there are similar catch-all clauses roughly equivalent to *"void where prohibited by law"*.  For example, the *Single Convention on Narcotic Drugs*, has the catch-all clause *"Subject to its constitutional limitations"*:  

> [Article 36: PENAL PROVISIONS][1]   
1. (a) **Subject to its constitutional limitations**, each Party shall adopt such measures as will ensure that ... *bla bla bla* ...

...and anything in that list of provisions which conflicts with Canada's Constitution is not binding, or is at least would be subordinate to its Constitution.

And so on for all treaties with similar *void* clauses...

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Also some nations think it better to follow the general spirit of a treaty, rather than the letter of it should the letter tend to weaken the treaty's purpose.  In 2018 still classifying cannabis as a narcotic seems like dubious and obsolete pharmacology.  It's arguable that it's better to only just ignore any items *erroneously* still included, in lieu of ignoring or throwing out an entire treaty just because it contains any single mistake.

  [1]: https://en.wikisource.org/wiki/Single_Convention_on_Narcotic_Drugs#Article_36:_PENAL_PROVISIONS