Ask HN: US and UK debt-to-GDP ratio

https://ourworldindata.org/grapher/gross-public-sector-debt-as-a-proportion-of-gdp?tab=chart&country=GBR~USA

Why has debt-to-GDP ratio tracked so closely for US and the UK over the last 30 years? Is this coincidental or being driven by other factors? This isn't the case when you add in other G7 countries.