Because this is in MSA, modern standard Arabic, not dialect. Arabic is a VSO, verb subject object, language ;therefore عند need to be first.
I might be wrong but in Lebanon, I'm used to say for example سيث عِنْد and not the other way around, I would like to know if it's just a mistake.
Dialect is a bit different from fus7a. In algerian/lebanon dialect, I would normally say seth 3ando, but in fus7a, you must say 3and Seth, because fus7a arabic is VSO (verb-subject-object).
الله عليك شاعر,
You're right, you wouldn't hear it used like this in a real world, always like you said - Seth 3ando (...). This is Fus7a though, so formally this is correct.
This question structure is wrong
Is dialect always the other way around? And if not, will everyone understans fus7a?
yup, fus7a is more right!
It should be عنده because عندك means YOU have like addressing him directly.
خطأ لازم (عنده) ه is important
I advise against learning Arabic here, I am an Arab, and I found errors in the pronunciation
U can say سيث عنده