One reason is because I don't want to debug a variable-name typo that those defensive practices would have caught in ten seconds... and I'm sure people don't want to have their posts here ignored.
Besides that, I've learned more about Perl by having to take code that barely works and make it pass the strict test.
There's a minimum standard of posting here that we really ought to take into account. People should be able to pare down their code into the smallest snippet that illustrates the program. They need to provide the exact error messages they encounter. They ought to format it nicely. They should list what they've already tried, if applicable. If they're running CGI scripts, they must use Taint mode.
Given all that, I don't think typing an extra 15 characters per script is an unreasonable burden. Take taint mode, for example. I wouldn't hire any CGI programmer who doesn't use -T because it's his "valid personal decision". Sure, it may work now, but when things fall apart in the future, having saved a few minutes writing code will be forgotten.