Why VRRP require /32 netmask?

A virtual IP address, either used by VRRP, HSRP, GLBP, or CARP, is just that: an IP address. It does not have a subnet mask.

On a Cisco router, for example, you configure HSRP like this:

interface GigabitEthernet0/0
  ip address 192.168.1.2 255.255.255.0
  standby 1 ip 192.168.1.1

The same is true for VRRP:

interface GigabitEthernet0/0
  ip address 192.168.1.2 255.255.255.0
  vrrp 1 ip 192.168.1.1

On Linux I believe it is a requirement to configure all your additional IP addresses on a physical interface as a /32, similar to your VRRP limitation on Microtik routers. These are limitation imposed by the implementation, not by the protocol. Had the Linux or Microtik programmers implement the concept differently perhaps you could give any subnet mask or none at all. After all, it does not need a subnet mask since it can get this information from the physical interface.