Why is it sometimes difficult to start doing what one enjoys?

Depression. I'm on an extreme version of this. I used to be a big-time gamer, one of my favorite pastimes. But haven't sat down to play anything for more than several minutes ever since the middle of last year, been through grief, a breakup, a stint of homelessness, a layoff, even more family drama, and losing basically all personal connections and ways with which I feel I can cope. So yeah depression can make you not want to do a thing another version of you enjoys doing.
- You are not alone. Somehow we can survive and it shall get better... Hopefully soon
- damn brother, you've been through a lot. keep powering through, you got this - only good things ahead <3

There could be a million reasons, but here's one I'm familiar with - 'task inertia', resulting from ADHD, perfectionism, fear of failure etc

Often, guilt. Sometimes, distraction or the inability to prioritize.
And sometimes, not being honest with yourself that you find something else more enjoyable.

You may be describing something called 'Workload paralysis' which, in short, makes tasks daunting because they seem so hard to start. Usually it 'leads to' procrastination, doing tasks instead that are known and easier.
Doesn't help that modern society tries to make you feel bad for having this parallesis, making it cause this whirlpool of self doubt.
Sadly there is AFAIK only one medicine: start. No matte rhow slowly, no matter how insignificant or pointless it may seem, start. As soon as you spend time doing anything, you will get better at doing it. This counts for all tasks.

I believe it's a combination of what others here are saying. We live in a world of near instant gratification, getting going on something that requires effort without an immediate payout feels too much like work.
FWIW, I find it helps tremendously to schedule a day / time in advance. For example, I spend the whole week looking forward to "Wrenching Wednesday" so I can go play with my cars. And I actually go do it now, which is something I couldn't always say before.

In a world where everything is optimised to give you immediate rewards and happiness an act that is a bit frustrating to start with or that requires a modicum of effort will lose out to those temptations, unless you make an extra effort to stick to a schedule to rewire your brain to get used to it as part of a routine

A lot of people are saying depression (which is true) but I'd also like to throw in stress. I like reading and playing strategy games, but I'm sometimes so stressed that I can concentrate enough to do these things. It helps to have some easy, no brain hobbies as well to help get started with relaxation, so you're able to enjoy your main hobby after you feel a bit better