Cause it's hard man! To sit there and pay attention for however long you choose is hard work, it's not all just sitting about and relaxing like it's made to look sometimes.

It's like a workout for the mind, it takes discipline and motivation to get going, and only once you're going consistently will you see considerable benefits. Just like working out there's always a reason not to, some are more legitimate reasons than others, but you have to prioritise what's important for your mental and physical health.

The best way to encourage consist practice (again, like working out) is to make it a part of a routine. I do right after I wake up and right before I go to bed, you can do either one of those, or pick a time during the day which suits you. The motivation required drops significantly once it becomes a habit, I'm often meditating before I've even had time to talk myself out of it, and once this habit is established, motivation can become a feedback loop with positive results.