Because it’s thick/sets fairly quickly so when stirring vigorously you are adding in oxygen bubbles.
When I work with resin I tap tap tap tap to encourage the bubbles up to the surface, obviously can be difficult depending on the shape you are working with, but that’s the trick I was taught in school !
Most casting is either done with resin or molten metal; and it happens that resins are “thicker” and have a harder time letting bubbles of gas filter up during solidification. Liquid metal has a “consistency” more similar to water than something like honey, which is what you see with resin. Metal castings also have to deal with dissolved gases, which is a big part of why cast objects have a reputation of being weaker than forged or machined metal objects – there are potentially gas bubbles in a cast object which can make it weaker. Casting is a process which is inherently poorly controlled, so issues like bubbles are something you need to spend a bunch of effort trying to combat.
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