1- Since web 2 emails like Gmail are the most used email service, it is also the most targeted by hackers
2- If you use gmail as your email service, you probably use Google's other services, so if you're hacked, you could have more information than you expected obtained by hackers
You can solve these mistakes by using a lot of accounts if you want, using each one for specific things, this reduces the chance of you having too much information leaked at once.