If you compare two people, you can say 'one is taller than the other is'.
But here the author was comparing an 'urn' with a 'man'. We don't normally say that a man is high. But here the author used high(er) for the urn, and therefore it is more clear to say that the urn is 'higher than a man is tall'. 'The urn is higher than a man' would imply that we would use 'high' to modify 'man', which, as I said earlier, is not something we normally do.
瞎說
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