DNS records are statically typed. An “A” record can only contain an IPv4 address, just like a variable of type “int” in the C programming language only can contain an integer. An MX record, just like CNAME, SRV, and others, can only contain a domain name, not an IP address of any kind. You can input a domain name which looks like an IPv4 address, but this is simply a syntax overlap; it happens to be that the normal textual representation of IPv4 addresses are also valid syntax for an unrelated domain name. Tellingly, the syntax for IPv6 addresses does not overlap with domain names, so you cannot make this error with IPv6 addresses.

So an A record should also hold domain names? Or IPv6 addresses?

In DNS, there is actually a single record type that should hold a physical address, for each supported protocol address. We have A records for IPv4, and AAAA records for IPv6. Anything else should be a domain name.

Adding both IPv4 and IPv6 support to every other record type would be a waste.

Note: ignoring other protocol addresses above to keep it focused on what is currently is large scale use.