Lego made some pieces that can't be pulled apart

Some would argue that the whole point of Lego is that the pieces are re-usable. Others might not care as long as the pieces can be used to make a cool-looking model. In any case, Lego's new 75313 AT-AT includes sub-assemblies that are "impossible to dismantle without resorting to using sharp metal implements," according to Brickset.

Take a look at the following, which is part of the walker's legs. The orange pin is inserted into the end centre hole, then an axle connector is attached to it. No problems so far.

Then, a green pin connector is positioned in the gap to the left of the axle connector before a 4l axle is pushed through it.

As a result, the assembly cannot be dismantled conventionally: there is only one way to do it. The green pin connector has a slit in it large enough for a knife blade. It's just about possible to use one to move the 4l axle fractionally, just enough to grip the end of it. However, it comes at the cost of damaging the axle.