Private key/address on those website is generated on-demand (basically when you access the page). I would recommend you try clicking button "Random" or "Last" on that website.
One last question. Is it more secure to generate a 24 word seed using dice/coins (manually) OR using CSPRNG present is software like electrum etc ? Which is more secure?
People usually would say both option are secure enough. Although it's hard to say which one is more secure since there are many variable involved needed to answer the question (such as quality of the dice or whether you call CSPRNG just after boot on device with very little activity/input). But FYI, Electrum actually utilize CSPRNG provided by operating system through function
os.random()[1]. Electrum would use
/dev/urandom on linux/unix-based OS and
CryptGenRandom() on Windows OS[1].
[1]
https://github.com/spesmilo/electrum/blob/4.3.2/electrum/wallet.py#L433[2]
https://docs.python.org/3.10/library/os.html#os.urandom