Because it is not mandatory.
The typical Latin word order is SVO, but not mandatory, and when the verb "sum" (esse) is used, it comes very often at the beginning or in the middle of the sentence.
When sum is used as the Substantive verb, it regularly stands first, or at any rate before its subject.
Est virī māgnī pūnīre sontis. It is the duty of a great man to punish the guilty.
The one normal exception to this rule [SOV rule] in Latin is when the verb “to be” (sum, esse, fui, futurum) is used. The sentences will mirror English word order (SVO). For instance: S V (O) English – The cook is in the kitchen. S V (O) Latin – coquus est in culina.