The short answer is, companies produce what most people are willing to buy and produces most income. That is not the same as producing best products for customers.

Some companies found the way to produce most income by offering best products for customers.

I am not an expert on kettles and might of course be oblivious to some glaring obvious facts about kettle-making business.

But it seems that it is very likely that good kettles just don't differ much form mediocre ones, when put on the shelf of the supermarket or when put on Amazon against competition that can buy a bunch of fake reviews. There would be no practical reason to spend a lot more on research to produce something that would not at all bring more income for the company.

Most people will buy cheapest kettle that satisfies their basic requirements. For me this is: it keeps proper volume of water, it heats water quickly, does not smell, is easy to clean from scale. Mine is quite expensive compared to most kettles only because "does not smell" is very difficult to meet (or it was when I bought it a decade ago).

There are good kettles. These are produced by companies who found their niche producing for customers who just don't care about how much stuff costs and will buy the best of each category regardless of how expensive it is. So while they might be good mechanically, I think overall they are not as good products when comparing return on the investment.

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Now, as to filters, I would NOT buy a kettle with a filter.

Filter is something that keeps impurities and I definitely don't want that sitting hot in my kettle.

Also, filter will not prevent scale from depositing. There is no practical way to prevent scale depositing other than using distilled water which will make everything taste awful and will probably not be good for your health.

Contrary to popular opinion, "hard" water with lots of minerals is better for your kettle than water with little minerals. The scale that deposits from hard water is very porous and it practically cleans itself and is easy to remove with a little bit of citric acid.

The scale that deposits from water that has little minerals, depending on composition, can be very dense, hard and nonporous making it difficult to clean.