Why in my iPad safari clear its memory often?

See I have a problem with my Safari in my iPad. The problem is that the Safari clears its memory by its own. For example take WhatsApp web in my iPad when i login in it using QR code as we know after few days my whole login data is gone and I have to login again and also take google for example when i login in it after few days like WhatsApp I have to re login as my Safari is losing memory. Please help.

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