Mainly because it will cause a performance problem with any insert. Maybe you won't see it on the first day, but the poor schmuck who has to support this a year from now will likely curse your name several times a day.
...and a debugging problem.
...and you are unnecessarily adding a point of failure to your day-to-day transaction processing.
These should be your mantras when designing triggers:
1) All triggers should be written to handle multi-record inserts.
2) The scope of a trigger should be limited to its table, if possible, and to its database, at the most.
If it's not practically useful, then it's practically useless.