Because there is a rule about not putting two consonances together. The v in tov and the σ in σκύλο can't be next to each other. This is from my Greek wife who was an elementary teacher in Athens.
Truth is, the rule has changed now and is considered wrong to omit -ν on masculine articles so that they are not confused with the neuter article...
Why is it 'το σκύλο τους' and not 'τον σκύλο τους'? Σκύλο is in it's masculine accusative form it seems but το is neutral...
Well, in my opinion, it's not. It's not completely wrong, since no one can barely notice it in speech. Natives usually don't pay attention to such linguistic details.
In this case, the final -ν would actually be preferred in a written text, since the rule says it should always be kept for the male gendered articles, in order not to be confused with the neuter ones. ^.^
Condensing what all of our instructors have been saying up to this point, the new "rule" is to keep the "ν", but it is not necessarily wrong to drop it, because many Greeks still do it the old way. And more importantly, we have to understand that we will see it both ways, and neither is per se correct or incorrect. As students, we should keep it to stay in step with the modern trend. However, some places in this program still have it the old way, but this should be corrected when the new tree launches.
Νο, it is a controversial point to keep or not to keep among the linguists in the accusative article το(ν). Because there could be confufusion with the neuter το in some cases. If there is ambiguity, you can keep it. Source: Greek wikipedia in το τελικό ν. https://el.m.wikipedia.org/wiki/Τελικό_Νι_(γραμματική)
For those interested in a brief overview in English regarding controversies about modern Gk and the intense feelings about the language, I recommend Peter Mackridge, The Modern Greek Language (reprint 1992) pp. 6-14, where he gives a brief history of what he calls "The Greek Language Question," and introduces "demotic," "katharevousa," and SMG (Standard Modern Greek). Apparently riots broke out in 1901 when a demotic translation of the New Testament was published (p. 9). Also helpful is chapter 11 "Style and Idiom."