Err, how would it know? /alias is a command, so you can't just assume "a line beginning with 'alias'" is the start of an alias. ditto for events, since /on could be a command too (and /ctcp certainly is). Example:
alias myalias {
if (%x == 2) {
echo -a x is 2
}
alias /myotheralias /echo -a hi
This would be reindented as:
alias myalias {
if (%x == 2) {
echo -a x is 2
}
alias /myotheralias /echo -a hi
; missing } is here
As in, /myalias would create a new alias /myotheralias. This is a valid usage.
mIRC would complain that you're missing a } at the last line, and its guess would be as good as it could get. The above code is perfectly syntactically valid, and *COULD* have been what your intention was. It's impossible for mIRC to read your mind and tell where your alias is supposed to end if you don't tell it.