Because IPv6 doesn’t fragment. This decision was made in order to save CPU power on routers. It has nothing to do with Mikrotik - it is an IPv6 “feature”. That size of 1280 is the largest possible packet, between these two hosts, given your actual network setup. Every time a connection is made, it is tested, to see the maximum packet size without fragmentation.
There is a substantial difference in how fragmentation works in IPv4 and in IPv6. Whereas in IPv4, any device along the path may fragment a received packet if fragmentation is not expressly prohibited by means of the DF bit, in IPv6, the behaviour is closer to TCP MSS handling - if a packet doesn’t fit to MTU somewhere along the path, such fact is reported back to the sender endpoint and it reduces the PDU size so that it would fit, i.e. it accommodates the PDU size to avoid fragmentation further on the path. In case of IPv6, the sender fragments the packets already when sending them if it sees that as the most appropriate way of handling the situation. See more e.g. here.