So we have 'to' form verbs, right? And they end with 'ar', 'er', or 'ir'. When we want to change that verb to 'he/she + verb' we remove that 'r'. For example the word 'hablar' to make it 'he speaks' you can just say ' habla' and another is 'hacer' but this time ending with 'er'. It will become 'hace' if we want to say he/she speaks. But it is kinda different with 'ir' verbs. For example 'venir' will become 'viene' instead of 'veni'
edit: I know it's kinda stupid to ask why because it sounds like I'm asking "why spanish is like that?" but actually what I mean is "why ir verbs different from ar and er."