They must assess their resources before creating new accounts on other social media because even they don't spend money for advertisements there, they will have to spend human resources to handle those social media accounts.
It does not help anything by creating accounts and let them unlively with no posts and activities but it has risk of account hacks that can be used to spread phishing links for example. If happens, it damages the business reputation and harm users.
Exactly, that's probably the reason why they are hesitant about it because if you want your presence in every social platform, you will need people to manage those things for you, you need to have regular posts and updates and that means you will also need someone who would create and schedule posts and updates on all those platforms otherwise there is no point in creating the accounts if you can't keep them active and running, and this can be resource extensive thing to do, IMO.
When platforms grow bigger, they tend to spread their wings wider because then they can do it without any problems since they will probably have the budget and the capacity to increase their reach by hiring more people and allocating more resources towards building a larger community around the internet.