Why can't we give block ciphers larger keys? I have limited math's knowledge so sorry if this question sounds stupid.

I saw many people complaining about AES, Twofish and Serpent that these ciphers all could be crackable in the near future and even today with big datacenters. 
But all these ciphers use max. 256 bit keys . And because there is no better way of finding the key then brute force the ciphers are equally strong. 

Would it be possible to take these algorithms e.g AES and just change the key size from 256 to 512 or 1024? I mean the algorithm is well tested, you would just have to modify the key and block size.
I saw many people complainig about AES , twofish and serpent that these ciphers all could be crackable in the near future and even today with big datacenters .


This is a good example of why we should always ask for citations and explanations, rather then just accepting what people say at face value with no scrutiny. These claims are about as far from reality as possible.


  But all these ciphers use max. 256 bit keys . And because there is no better way of finding the key then bruteforce the ciphers are equally strong .


These 3 ciphers all have similar if not identical parameter configurations because they were all designed for the AES competition. It is true enough for practical purposes (but not exactly so) that all three are equally strong and that the only way you'll break them without side channel attacks is via brute force. 

Which is to say that you won't break them, at all.


  Would it be possibel to take these algos e.g AES and just change the key size from 256 to 512 or 1024 ? 


It is not possible to just increase the key size for an algorithm like AES; There are other aspects that are derived from the size of the key that you would have to take into account and modify as well. For instance, the AES key schedule (which is part of the AES block cipher) of AES-128 and AES-256 differs.


  I mean the algo is well tested , you would just have to modify the key and blocksize . 


Ok, let's concede and say we construct such a thing, and also assume we don't somehow introduce additional points of failure while doing so: What problem have we solved by increasing the key/blocksize? It was already impossible to search. Perhaps this is a good question to ask those people that were complaining about how easy it is to break AES, twofish, and serpent...

Why can't we give block ciphers bigger keys

We can, we do not need to. You can always construct make an algorithm use more space and time. The goal is to use the least amount of resources possible to get the job done. Once you understand this, it will be clear why we do not use massive keys or hundreds of cipher rounds.