Because it's a "false" information : even when settings this, there are a lot of server where it can not work.
Better to use apache/PHP configuration (for server admin). But you can use session DB (in your config.php file) to have again the option.
Assistance on LimeSurvey forum and LimeSurvey core development are on my free time.
I'm not a LimeSurvey GmbH member,
professional service on demand
(or search sondages pro).