Not on the first 2 because they're illegal for minors. Yes on the last 2. A parent can e.g. forbid their minor from being employed if it's hindering their studies. They also have a say on their romantic partner and how they interact. If rules are not followed, they can e.g. be grounded.

Generally, the reasons are:

1) The parents know their child best (as opposed to a lawmaker, a voter, etc.); so, they can e.g. make the best decision whether their child is responsible enough to engage in a given activity, like social media and the internet as a whole. This is just a matter of fact from living together throughout the minor's life, as well as knowing themselves and what traits the child may have inherited.

2) Generally, nobody is going to care more about the child's welfare than the parents. They're the people you can most count on defending their child's best interest. Other people may just have other agendas. In the case of what we're going through with social media, the parents may determine that the internet access they provide is overall beneficial to their child, while attempts to forbid it may lie entirely in furthering a surveillance state of the adult population which their child will eventually be a part of. Children don't stay children. One must also think of their adult lives when advocating for them.

3) Families differ a lot in values. They each have their own perspective on the proper way to raise their child. There's no consensus on a lot of things both big and small. The views of an individual family are generally going to be more stable and consistent than whatever's going on in the general political arena and changing culture. They also depend a lot on the family's individual circumstances, which the child will also exist in and may need to navigate as their parents have done. The child may also inherit traits from their parents and may need their particular guidance based on that. Again, in the case of social media, the child will likely benefit from guidance on e.g. how to not use it compulsively, how to protect their privacy (including being watchful of how much of themselves they share, to not depend on their own obscurity, etc.), how to respond to other people, not take things personally, not need others' validation, why are parts of the internet the way they are, etc.

4) Once the minors are adults, the parents will ultimately have no say and no obligation. One needs the opportunity of the time they're minors to be able to gain that guidance. If the parents are disempowered to make such decisions, it's ultimately harmful to the child. They will have to parent themselves.

The fact that you asked this makes me think that either you're an adult that feels badly towards your parents (maybe justified or maybe not), or you're a minor that doesn't understand why parents have authority. In case of the latter, you might think that another authority would be better. You might have idealisms (e.g. on the ability of something as broad-brush as law to substitute parenting), and simply lack the experience/perspective to see why such idealisms fail.