In RFC 5798 which defines the operations of VRRP, you will find in section 7.3 the following:
(emphasis mine)
7.3. Virtual Router MAC Address
The virtual router MAC address associated with a virtual router is
an IEEE 802 MAC Address in the following format:
IPv4 case: 00-00-5E-00-01-{VRID} (in hex, in Internet-standard bit-
order)
The first three octets are derived from the IANA's Organizational
Unique Identifier (OUI). The next two octets (00-01) indicate the
address block assigned to the VRRP for IPv4 protocol. {VRID} is the
VRRP Virtual Router Identifier. This mapping provides for up to 255
IPv4 VRRP routers on a network.
IPv6 case: 00-00-5E-00-02-{VRID} (in hex, in Internet-standard bit-
order)
The first three octets are derived from the IANA's OUI. The next
two octets (00-02) indicate the address block assigned to the VRRP
for IPv6 protocol. {VRID} is the VRRP Virtual Router Identifier.
This mapping provides for up to 255 IPv6 VRRP routers on a network.
So as you can see the mac address convey information, it tell the other devices that this is a VRRP address and even provide the VRID.
Also keep in mind that you can define multiple instances of virtual gateway on the same device, and while they could all use the same mac address it is cleaner to have separate mac-addresses.
The same logic apply to HSRP (which, admittedly VRRP is derived from).