This is a common minhag whose source is not so clear. It can be explained based on the following.
The gemara in Brachos 21 states that if you come in late you are not allowed to start שמונה עשרה if you cannot finish by kedusha.
There is an interesting dispute relating to this halacha. What should you do if somehow you are in the middle of שמונה עשרה and the ש"ץ reaches kedusha. Rashi in Succa says that you should stop saying שמונה עשרה and listen to the ש"ץ and be יוצא שומע כעונה. Tosafos (in Brachos) has 2 questions on this:
1. If this is true then why aren't you allowed to start שמונה עשרה at any time, you aren't losing anything, you are יוצא קדושה through שומע כעונה anyway?
2. If you are יוצא why isn't it a הפסק ?
Tosafos answers 1 by saying that although you may be יוצא with שומע כעונה it is better to say the words (this needs explanation). Tosafos doesn't answer the second question.
We can explain the dispute based on the following. There are 2 ways to look at שומע כעונה:
1. Take the words literally, the halacha is saying that if you listen the halacha views it as if you said the words
2. You are יוצא with his דיבור meaning that you didn't say the words, however the halacha says that you are יוצא by someone else saying the words.
Tosafos would hold like 1, therefore it would be a הפסק because the halacha views it as if you said the words, so you said kedusha during שמונה עשרה. Rashi however holds like 2 and therefore there is no הפסק, you said nothing you are יוצא with his דיבור so it is ok even in the middle of שמונה עשרה.
This is the pshat here by the עשרת בני המן as well. The Rogatchover says that although the whole megilla you are יוצא with שומע כעונה you can't be יוצא the minhag of saying the names in 1 breath with שומע כעונה. Why not? It would seem that the Rogatchover holds like Tosafos, and therefore שומע כעונה makes it like you said the words but it can't add anything over and above that, so you said the words but not in 1 breath. However, according to rashi you are יוצא with his דיבור and his דיבור was good (in 1 breath) so just like he is יוצא with his דיבור you are as well.
The same question comes up as well by bircas cohanim. Can 1 kohen make the bracha and all the other cohanim will be יוצא with שומע כעונה? The Beis Halevi said no because bircas cohanim requires קול רם that everyone in the shul needs to hear you. It would seem that the explanation is the same as for the עשרת בני המן that שומע כעונה can't create קול רם.