Existentialists do not necessarily posit that life is meaningless. There stance is that life lacks a pre ordained, god-given meaning and that each of us is responsible to find his/her own meaning. So, life can be meaningful, but that meaning is not served on a silver platter. There's a fine nuance there that makes a big difference.
Personally I have not found any purpose to my existence, but I do not deny that other individuals, who are better equipped than me, enjoy meaningful lives.
The reason I "subject" myself to living, as you put it, is that suicide is an extremely difficult undertaking which requires both mental and practical preparations.
The answer to your question (as far as I'm concerned) is: Killing oneself or having a cup of coffee are not equal, even for someone who has failed to find meaning to their existence. The latter is a delight, the former is the hardest thing you can to.
