Sure it's better to do all juices by yourself. But do you make your own mango juice, passion fruit juice, pear juice, peach juice, ... ? You could even make your own grenadine.
The thing is, everyone can squeeze a lemon or lime. Not everyone has a juicer at home (to get a good result, blending and filtering isn't enough.. or you filter it multiple times which needs some time..). Or wants to clean the whole thing for one small cocktail.
Besides that, you have to get a perfectly ripe fruit. Else it will taste not so good. The advantage of bought juices is that they can harvest the fruits riper than the ones we can buy (non-fly-ones) because the distance to the factory in general is far shorter than to your supermarket.
So, all in all, I'd say it's great if you make your own juice. But I haven't seen a lot recipes that list "fresh pineapple juice" or smthng like that yet. So imo this is nitpicking and no reason to classify it as bad. Whereas bought lime/lemon juice is a reason.
