is Bitcoin development so difficult?
Of course it is. That's why we have more theoretical concepts, than practical implementations of many things (for example decentralized sidechains).
Is it a thing from the past?
Well, block space is more expensive, than it was. In the past, everyone assumed, that we can load a lot of stuff on-chain, and don't worry about it. And many services worked well, as long as the chain was not congested. But when the fee market started to form, then people stopped pushing everything on-chain, and started thinking about other solutions (which is not easy).
Nobody writes bitcoin-related software anymore?
It is easier to pump and dump a new altcoin, than to contribute to Bitcoin. And it is more profitable to do so, if you are the creator.
Why on earth isn't there any good bitcoin-related product advertised in the forum?
Because if a product is "good", then it doesn't need additional advertising. People will simply notice it on-chain, and join it.
And if a product is "bad", then you have to convince enough people, to pump it properly, and then you can dump everything, when it becomes profitable, to just sell all of that, and cash out, by selling all of your easily-obtained ALTs for BTCs.
Is it because bitcoin projects can't be easily monetized?
If you have an altcoin, then it is distributed from scratch, and created out of thin air. Creators are then early adopters, who can early-mine or pre-mine a lot of coins, and then sell them.
On the other hand, if you have "Bitcoin project", then it is 1:1 backed by BTCs. Which means, that you have to stick to the rules of Bitcoin. Which is hard, because you cannot just program anything you want (because your changes have to be accepted by the rest of the network, and you need consensus for that), and also, you have to buy real BTCs (which is expensive).