That would be like saying "How hard can it be to type a program into a computer?". Statistical process control, polymer science, release agent chemistry, die metallurgy / thermal / fluid / mechanical design, etc are large and deep fields.
There's probably a better video, but here's the official one about how Lego are made.
Actually, the video suggests that the complicated part is not the baking process itself, but the handling of thousands of blocks of different types and colors.
The logistics are obviously challenging too. But their process is a trade secret, so of course the video glosses over it. Having thousands of block types also means solving the challenges I listed thousands of times.
It seems obvious that the real question that was being asked was "How hard can it be to bake a piece of plastic into shape [given the current state of the plastics industry and technology in general]?" and the answer to that question is "not difficult at all."
If you consider everything that goes into setting up an email filter in your mail app, starting with how transistors work but also including ocean cables and the liquid crystals of your display, well, sure, it's crazy how much is needed just to drop a couple of emails into the right folders... but outside of a science documentary, that's not really what people have in mind when they ask whether something is difficult.
It seems obvious that the real question that was being asked was "How hard can it be to type some computer words to make a program [given the current state of the software industry and technology in general]?" and the answer to that question is "not difficult at all."
Sure, you're not reinventing computer science. But a lot more work goes into making and maintaining a product than setting up an email filter.
The product is conceptually simple, but nailing the press fit on every block has got to require tight quality control- which is something fly-by-night knockoffs have never been known for.
The problem is the same as in lots of mechanical engineering designs - accumulated tolerance.
Imagine building a wall that is 100 blocks long with lego bricks. In order to fit well, the length of the first row has to be close to the length of the second row (etc.). Otherwise you will have the feeling of forcing it into place (if it's even possible).
That length difference is the accumulation of the error on each of the 100 bricks in a row so to get a good fit each brick needs to be made incredibly precisely. The required tolerance is approximately 1 micron.
Injection moulding is a surprisingly imprecise art (if you're not careful), so making them repeatably to that tolerance at high volume and low cost is a difficult skill. I can't find the reference now, but Lego has developed the skill to the extent that in the past they have been asked to advise defence companies about precision manufacture.
I recently got some Mega Construx for my kid, as they have the license to make Pokemon characters. One of the Pokeballs that came in the set has visibly deformed pegs on the top, making it impossible to use as displayed in the instructions. Luckily it's not a big deal to be able to stack things on top of the Pokeball so I didn't bother to return it for that.
There are plenty of clones. MegaBlocks has been the most visible for a long time. I believe Lego owns the exact dimensions and ratios of their bricks.
I recently got into NanoBlocks, which are about 1/4 the scale and different dimensions. Ease of storage and display due to tiny size is a big differentiator
Its not hard, it just costs money. Lego is the default option and the only reason you would pick another brand is if it was cheaper and to be cheaper they have to cut corners since Lego isn't massively ripping people off.