I agree, mobile wallets are very convenient nowadays, but it is not necessary for every project to have its own wallet, you can use, for example, Trust Wallet, which supports all coins.
Are you saying that trust wallet supports all coins that's available in crypto space? You are wrong, trust wallet only supports some coins, I think like up to 100+ different coins but we have thousands of coins today, isn't it? OP is talking about fresh projects with own blockchain, it takes time for such projects to get listed on trust wallet