1 SFX: Clunk! {Prof. Jones hits something with his pick}
2 Prof. Jones: I've found something! Help me dig it out!
3 Monty: It's wood. Unlikely to be Arthurian, 1500 years old. It's too well preserved.
4 Prof. Jones: Well that just goes to show it probably has magical properties!

The decay of wooden objects over archaeological time is actually very complicated, and it's difficult to make blanket statements about any particular piece in isolation and without a detailed study. (Monty is no doubt using his legendary level archaeological skills to produce such a quick analysis.)

Wood is subject to many different forms of decay, caused by various biological and chemical agents. The rate and nature of decay depend heavily on environmental conditions. Wooden objects in dry, desiccating conditions can survive thousands of years, while wood in moist environments that promote growth of bacteria, fungi, and wood-boring animals can be completely destroyed within a few decades. Chemicals such as salt and other things dissolved in groundwater can also have effects ranging from killing attacking organisms, to chemically breaking down the cell structure of the wood. Sometimes wood is even penetrated by minerals and fossilised as petrified wood.

There are hundreds of scholarly articles on the archaeological decay of wood and the effects of various agents^[1], but here's a nice overview with examples.

[1] Disappointingly, almost none of them mention the influence of magical effects.