First, I could myself generate a seed phrase without anything. I just pick 24 words from the list given in the BIP 39 protocol.
So if Ian colman give me a list of 24 words randomly in the same list what can be wrong? It is just picking word at random.
Picking random words yourself from the word list is widely recognized as highly insecure and one of the worst possible ways to generate a seed phrase. This is also absolutely not what Ian Coleman's software is doing. What Ian Coleman is actually doing is generating a pseudo-random number via
crypto.getRandomValues and then encoding that in to a seed phrase. Your browser is simply not a very good source of entropy for this process. Compare this to Bitcoin Core, for example:
Most good wallets will be based on entropy directly from the OS and the computer's hardware. Bitcoin Core, as an example, draws entropy from /dev/urandom (which is from the OS, or the equivalent on non-Linux systems), RDSEED/RDRAND (which is from the processor), and a whole host of data from the computer itself, such as current resource usage, timestamps, kernel parameters, network data, version data, etc. All of this is then combined through a variety of techniques such as XORs and hashes, so if one source of entropy is weak or compromised then your final result should still be secure.
It is fine to not fully understand the process behind seed phrase generation or the risks which need to be considered and mitigated against. Most people don't. But you should at least try to realize this and stick to the tried, tested, and recommended methods instead.
If you are dead set on using a webpage to generate your seed phrase then ultimately, we can't stop you. But you are taking on unnecessary risk by doing so, especially when there are much safer alternatives available.