I asked the UK Department for Transport essentially this question earlier this year and got the following response:
The DfT is responsible for regulating transport operators to undertake security measures aimed at protecting passengers, the mode of travel and associated infrastructure from acts
of violence, including terrorism. For international rail travel, statutory powers are provided by the Channel Tunnel (Security) Order 1994.
The Government takes the security of the Channel Tunnel and the protection of all those travelling and working within that network very seriously. The security measures that are in place are designed to protect the travelling public and the Channel Tunnel from unlawful
interference and possible attack.
Responsibility for the security of the Channel Tunnel is split between the UK and France. Trains and shuttles travelling from the UK to France are the responsibility of the UK, and the DfT sets the security measures that both Eurostar and Eurotunnel must undertake. We continuously monitor the effectiveness of security measures with the aim of improving our controls, taking account of new and emerging threats. This is done through commissioning research projects and acting upon recommendations.
Trains and shuttles travelling from France to the UK are the responsibility of France and security measures are undertaken by the French Customs authority, the Douane. We work closely with French authorities to ensure the comparability of security measures
deployed at both stations and terminals
The protective security regime in place is based on the assessed threat level in both the UK and France, and is designed to be proportionate in mitigating the security threat to the Channel Tunnel, whilst causing the minimum inconvenience to passengers.
Therefore, it would be inappropriate to be specific about the measures currently in place. However, it is worth emphasising that not all our security measures are visible. Every passenger, regardless of whether they are travelling from France or the UK, has an equal
chance of selection for screening, which gives the regime a deterrence value.
I hope this helpful, assuring you that the Department takes the security and safety of the travelling public seriously.
My hypothesis for the real reason is that when the relevant laws were passed, they were following an 'airport style' security model and not a 'train style' model. As they are encoded in law with responsibility split between UK and France changing it will be a hassle and there is no political will to do so.